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The Birthday Bang Theory

Summary:

In which Senku has a special birthday present in mind for Tsubaki.

Notes:

Disclaimer: This fic is post Stone-Wars arc, so waaaaay ahead in the Savage Daughter timeline. I've made it as non-spoilery as possible, but if you want to be extra safe, you're more than welcome to turn back now.

If you don't mind some minor spoiling, read on!

(See the end of the work for more notes.)

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“Now, if that’s all for today’s strategy meeting-.”

“Ahem!”

Damnit, he knew this would happen.

Senku gave Gen a side-eye; the mentalist only gave him an expectant smile in return. All day Gen had been like this, dropping not so subtle hints for the scientist. “You DO know it’s Tsubaki-chan’s birthday next week, right Senku-chan?” he’d asked. “It’ll be the first birthday you two will share as a couple, Senku-chan!” he’d cooed. “I mean, it would be just rude to NOT do something special for your baby mama, now wouldn’t it, Senku-chan?” he’d said behind a sleazy smile. Bringing it up now, right in front of the other “generals”, was just salt in the wound at this point.

As if I haven’t been stressing- er, strategizing about her birthday for a month already!

“Okay, fine,” Senku sighed. “There is one more thing.” The scientist silently vowed to reinvent itching powder and douse the mentalist’s entire wardrobe. “I have been informed that next week is Tsubaki’s birthday-.”

“Eally-ray?!” Gen squealed, as if he didn’t already know.

“...And as for what I plan to give her, I have three possibilities. Would it be okay to run them by you?”

“Of course you can,” Ukyo said with a smile.

“You don’t have to ask us twice!” declared Chrome.

“Aha, yes, of course!” Ryusui exclaimed. “If our fair Hippolyta won’t accept a gift from me, then I shall simply make your gift to her just as worthy!”

Don’t throw Sulfurina at him, don’t throw Sulfurina at him, DON’T THROW SULFURINA AT HIM!

Instead of flying into an illogically jealous rage, Senku inhaled deeply through his nose and reminded himself that Tsubaki was with him, not Ryusui. “So, here’s Option 1. We modify the carbon fiber from the Steam Gorilla and use it to make some badass paper armor!”

“I WANT IT!” Ryusui proclaimed immediately, hands slamming down so hard into the meeting table that he managed to lift himself.

…Are we 10 billion percent certain we can’t just chuck him into Sulfurina’s lake?

Ukyo was kind enough to pull the captain back down with little fuss. “As cool as that would be, this seems like a project for the Battle Team as a whole. And maybe making something battle ready after everything that’s happened between you two… would be in poor taste.”

Excellent points. Senku scratched that off his mental birthday registry, but stored it in another folder for later. “Alright, Option 2 is technically a gift for Mirai and the baby too. Tsubaki mentioned liking “The Little Mermaid” when she was younger, so maybe commissioning our resident mangaka to remake a book version of the story? It wouldn’t be by me necessarily, but as long as it makes them all happy, that’s fine by me.”

“Aww, how eet-sway!” Gen cooed. “A bedtime story for little baby Shishio, and the thing our elder Shishio sisters bonded over.”

“Yeah…,” Chrome said sceptically. “It’s a pretty good idea, but probably better for Christmas, you know? This is supposed to be just for your girlfriend, after all.”

More good points…. “Then I guess it’s just Option 3 left.”

“What’s Option 3?”

“That I have coitus with her.”

The reaction was immediate. Gen was, for once, at a complete loss for words. Ukyo’s eyes went as wide as Suika’s lenses and his face was tomato red. Ryusui keeled over in a great belly laugh, pounding his fist on the table. Chrome… he just stared at Senku confused. “Co-oy-tus? What’s that? Some kinda baaad chemical?”

“I-It’s, um,” Ukyo stammered. “W-well, Chrome, what h-he means… What Senku’s talking about.”

“It’s sex, Chrome,” Gen deadpanned. “He’s talking about sex.”

Ryusui’s laughter somehow doubled.

“Oh, that’s all? I mean I guess- YABEEEEEE?!”

Senku put his own red face in his hands. “Look, if you ignoramuses aren’t going to actually help-!”

“No no no!” The scientist felt a hand on his back and immediately knew it was Gen’s. “It was just… unexpected, that’s all! L-let’s go over our oices-chay again. So, we have armor thing, storybook thing-.”

“Wild thang!” Ryusui collapsed on the table, laughing at his own joke.

Surprisingly Chrome was the one to pull the captain off the table this time. “Knock it off, this is serious! Senku trusted us to help him!”

“And we’re going to!” Ukyo declared. “It’s just… Senku, this is a pretty big step for you guys, doing… that again after so long. Are you sure about this?”

Am I?

Senku had already run the what-ifs in his mind. What if their first time was some fluke? What if this time the sex was bad and she didn’t enjoy it, or what if he didn’t enjoy it or was just bad? Or what if it was good, but then the emotional vulnerability post-coitus takes over and-!

But then he thought of Tsubaki, love of his life, mother of his child, and the stress-induced tightness in Senku’s chest eased and all that was left was warmth. “I know that intimacy in any form is not my strong suit,” he said. “But I do want to show her how much she means to me, how much having her safe and back in my life matters to me. And I think now is the right time, and this is the right method.”

“Aww!” Gen, Chrome, and Ukyo coo’d in unison.

“If this is your desire,” Ryusui said, suddenly serious again. “Then we’ll help you see it done!”

Senku smirked. “Then it’s decided: Tsubaki’s gift will be my genitals.’

They lost Ryusui after that.

While the rest of the generals were picking the wheezing captain off the floor, they didn’t notice the pink blur hurriedly running away from outside their meeting hut.


All and all, Tsubaki’s day had been going fairly well. She saw Senku off this morning with a kiss on his cheek; their baby had been unusually well behaved for most of the day; Mirai had even offered to watch the child for an hour while the elder Shishio sister stepped out for errands. She’d agreed (only when Ruri had promised to keep an eye on both the martial artist’s sister and her baby) and Tsubaki had met up with Minami and Mei on their way down to the Settlement’s new makeshift market.

On the way down, the trio ran into Homura, who looked especially determined when approaching them. “Oh, hello Homura,” Tsubaki greeted and waved politely. “How are you-.”

“Bikini wax.”

….

“...Pardon?”

“Bikini wax. You should get one. I can help.”

That did not resolve any of the martial artist’s confusion. “Why… why would I need a bikini wax? It’s halfway through Fall and bikinis don’t exist anymore.”

Homura flushed red yet remained silent. The pink girl then waved Minami over and, once the reporter was close, whispered something in her ear. “Oh.” Tsubaki watched the blonde woman’s eyes widen and her face stretch into a grin. “OH!” Then Minami was hopping giddily from one foot to another. “Oh, this is just too good!”

“What?” Tsubaki asked. “What’s going on?”

Minami’s excitement fell away instantly. “I mean, I don’t want to spoil it for you.”

“Yes you do,” Mei deadpanned.

“Oh hell, YES I do!” Her glee returned two-fold. “Apparently Senku’s planning an extra intimate birthday party for you, if you know what I mean!”

What does she…? Oh…. OH! B-but she couldn’t possibly mean that… right?

The martial artist took a breath. That couldn’t possibly be it. She turned to Homura, face expertly neutral. “That’s not funny, Homura,” Tsubaki said after a moment.

“I heard. Senku’s exact words: “Tsubaki’s gift will be my genitals”.”

Oh my, oh me oh my oh FUCK!

Tsubaki was about as red as Homura’s and some sort of high pitched whistle was currently escaping her throat. The second she found herself crouched on the ground, Mei sprang into action. “Deep breaths, Shishio,” she commanded gently. “No need to panic.”

“I’m not panicking, am I panicking?! What does that mean, my gift is his… his… ACK!”

He wants to do it, but we haven’t done it since that night, and OH GOD what if it’s just like last time, or what if it’s nothing like last time, holy fucking shit, should I get a bikini wax?!

Mei’s hand was rubbing her back now. “Deep breaths, Tsu.”

…Right. In for four, hold for four, out for four, repeat. The air filling her lungs and the circles rubbed in between her shoulder blades were starting to calm Tsubaki. “Okay…,” she breathed out. “Okay.” She remained crouched where she was, but otherwise her body was hers again. “Oh God, I can’t do this.”

“Sure you can!” Minami declared. “I mean, you and Senku have already done it before. What’s stopping you from doing it again?”

“So much has changed between then and now,” Tsubaki said. “And I’m finally back in a good place, I’m happy! What if- what if doing… that again ruins things?!”

“Oh come on! What’s the worst thing that could possibly happen? And you’re not allowed to say “getting pregnant again”!”

“I was actually going to say that I could put myself back into such an emotionally vulnerable state that it leads me down a path of bad decisions and disagreements, eventually ending in me killing my lover. Again.”

“O-oh.”

“...Okay, but what are the odds of that happening twice?” Mei added.

The martial artist looked up at Homura. The pink girl’s expression was hard to read for most people, but Tsubaki detected a hint of regret behind her eyes.

“Sorry,” Homura said quietly. “Only wanted to help.”

“It’s okay, Homura.” Tsubaki held out a hand to her; the pink girl crouched and accepted the hand with a squeeze.

“He wants to show you how much you mean to him.”

…He does?

There was a faint fluttering in her chest now. A feeling Tsubaki that brought her back to those early days back at the treehouse, when she had first realized she was in love with Senku. She thought back to that one night before things soured, when she’d poured her heart out to him and he’d replied by pulling her into their first kiss. Tsubaki remembered that one blissful moment after that, where nothing else in the world- past or future- mattered but her and Senku, and…. And the product of that one perfect moment.

And now Senku wants to share that feeling with her again?

He wants to show me how much I mean to him….

“Hmm,” was all she said aloud.

“So what now?” Homura asked.

“... I… I suppose… since it’s a birthday gift, after all.”

From above, Minami squealed and clapped her hands. “Oh, this is so exciting! There’s so much to do, we’ve gotta get you ready, Tsubaki!” Homura opened her mouth. “And no, we’re not giving her a bikini wax.” The pink girl closed her mouth again. “But we will be getting you all dolled up for your birthday date! Yuzu can make you something to wear, I can see what I can scrounge up for make-up-. OH! And I can give you some tips that are guaranteed to drive your nerd crazy!”

Tsubaki was less and less sure about agreeing to this. She watched her reporter friend talk animatedly with Homura regarding her “glow-up”. Well, not so much talk as Minami listed her steps while the pink girl nodded or shook her head in return. So Tsubaki turned to Mei. “Well? You’re the doctor. What do you think?”

Mei shrugged. “Not my body on the line,” she said matter-of-factly. “But I’ll see what I can scrounge up for a round of Plan B. Just in case.”

“Thank you, Mei.”


The week leading up to her birthday passed by agonizingly slowly for Tsubaki. The worst part was that Senku was completely unbothered by it all! He acted as he always did when they were together, as if he hadn’t admitted to a room full of his friends that he wanted to… to… ugh, Goddamnit, she couldn’t even think the word without blushing!

There were some moments she could happily forget about the countdown to that “special day”. Spending time with her friends, her sister, her child; hunting side by side with Kohaku; working however she could on the ship’s construction. But even then there was that nagging fear- or was it anticipation? Tsubaki didn’t know what, not even on the morning of October 10th when Senku came up to her and asked “You feel up for a birthday date this evening? Around 9? Ruri’s already agreed to cover baby and Mirai.”

If the past week had been a slow crawl for Tsubaki, then this one day had been an eternity.

After endless hours of birthday wishes and small gifts here and there (Mirai had made a little seashell tiara for her and that was by far Tsubaki’s favorite), it was finally time. 60 seconds to 9pm. Minami and Yuzuriha had put her in a black blouse with sleeves that hung dangerously low on her shoulders, as well as a slit red skirt that barely made it half-way down her thighs. Homura had somehow managed to pull back her mane into something resembling a half up bun. Mei’s “Plan B” was all but burning a hole into the small leather clutch in her hand. She felt ridiculous, standing and waiting to knock in front of her and Senku’s own home, dressed up like some 18 Plus Barbie Doll.

9:00. Too late to turn back now. Tsubaki knocked.

“Come in!”

She stepped inside and found her entire living room changed. All the baby gates and play sets had been pushed to the side, and the kitchen looked cleaner than it had on the day she’d first seen this place. A table and two chairs sat in the middle of the floor, all topped off with a single lit candle and two piping hot bowls of ramen, because apparently her ridiculously wonderful boyfriend’s romanticism stopped partway through the candle-lit dinner trope and honestly Tsubaki was grateful for that. “Wow,” a voice said toward the direction of their room, and there was Senku, dressed in a red button-up shirt and black slacks, because of course he went to Yuzuriha and of course she made sure they would be matching! “Beautiful,” he whispered.

“Thank you,” she said. “And… you clean up nicely yourself.”

“Keh. Thanks.”

Neither of them moved. Tsubaki used all of her willpower to not wring her clutch. Senku brought a hand up to the back of his neck in some ancient habit.

“So….”

“So… dinner?”

“Oh, um, sure.” But Tsubaki’s stomach turned in… what? Excitement? Nerves? Either way, she didn’t think she could handle even a bite of ramen. “But… no offense, you’ve done a magnificent job with all this.” She gestured vaguely towards the table and ramen. “I think, however, it might be better… if we skip right to presents?”

She watched Senku’s bug out of his skull. “Oh, yeah, totally!” he said a little too quickly. “We can absolutely do that, it’s your birthday, after all!” He started for the table, froze, then waved a dismissive hand at the display and looked back at Tsubaki. “So, I-I should probably explain this gift.”

He’s nervous too. Maybe… maybe I can help?

“You mean before you….” Just as how Minami had taught her, Tsubaki twirled a lock of her hair and batted her eyelashes. “...Give it to me?”

Senku stared at her like she’d just suggested drinking Nital. “...Why are you doing that?”

“Doing what?”

“Acting like Minami and saying “give it to me” like that.”

Ugh, SCREW this and SCREW that reporter! Just suck it up and be honest with him! That’s what he likes about you!

“Senku…,” she began. “I… I know that your gift is… for us to be intimate again.”

Tsubaki watched as some of the tension disappeared from Senku’s shoulders. “...Oh. And how do you feel about that?”

How did she feel? She felt terrified. She felt exhilarated. She felt like things were about to go wrong. She felt this was where she was meant to be. Most of all, she felt in-love with this man all over again, because this was for her. This was to show her how much she meant to him.

So Tsubaki leaned forward and planted her lips right on his. Their kiss lasted only five seconds before Senku pulled away, a smile right where her lips had been. “That’s cute and all, Lily-White, but I’m afraid I’m going to need verbal consent.”

“Yes,” she breathed. “Yes, I would like that. Very much.”

“...Okay, good…. Good.”

Senku tilted forward for another brief kiss. Then he pulled away and now he was pulling Tsubaki towards their room. She let herself be led to their bed, and let her clutch be put on the nightstand. “By the way,” Senku said once he sat down beside her, then pulled out a clay jar from who knows where. “I managed to whip this up. I won’t bore you with all the science-jargon, but the important thing is… it’s a spermicide. It’s not quite as effective as condoms would be, but it’s better than nothing.”

“I see.” And Tsubaki did see, watched him set the jar on the nightstand, even heard the liquid swish inside it and just imagined how it would feel when it was… well…. “I also brought something. In that clutch.” She waved towards the general direction of the nightstand. “Mei made it for me to take… after the fact.”

“Gotcha. Doubling up on protection options will 10 billion percent help us in the long run.”

“Hmm.”

Then Senku was leaning up to kiss her again. Tsubaki accepted the kiss, grateful to not have to worry about forming words anymore. Her hands cupped his face, then slid across his shoulders and down his arms, and meanwhile his fingers started to unbutton her blouse while they kissed again and-!

Oh God, she thought, this is actually happening, isn’t it?

Senku pulled away, a million thoughts racing behind his red eyes. “...You don’t seem so sure now.”

“I… I’m just nervous, is all,” Tsubaki admitted, looking at anything besides Senku because oh God, he must be so disappointed. Why couldn’t she just do this?! “That’s stupid, isn’t it? We’ve already done this before, and-.”

“And a lot’s changed since then.”

“...Yes.”

….

“Tsubaki,” he said. “Look at me.”

She did.

“This… this doesn’t make or break us. We’ve already contributed to the global repopulation effort-.” That earned him a snort. “-so if we never do this again, or if that one time before was our first and our last… that’s fine by me.” One of his hands squeezed hers; she squeezed back. “I see you, Tsubaki. I care about you, and I want to make you happy. If this isn’t going to do that for you, then we can go right back out to the other room and figure out how to reheat that ramen.” Senku’s other hand came up to her cheek and brushed away a tear she didn’t know was there. “It’s up to you, Lily-White. You’re the birthday girl, after all.”

I love this man so much. I really, truly do.

For the first time that night, Tsubaki didn’t have to force a smile; it came to her all on its own. She reached up to hold his hand, planting a kiss on his palm. “I think… in words you might appreciate… we should continue the experiment. We’ll never know until we try again, yes?”

“Are you sure?” Senku asked.

“Positive, Love. Besides, I’d hate to let all those tips Minami pounded into my head go to waste.”

“Keh! You think that was bad? I had both Gen and Ryusui lecturing me through Intercourse 101! The things I heard there… I’ll never be able to look at the number “69” again.”

They both laughed, finally free of the awkward tension and nervous anticipation of the evening. Once their giggles died down, Tsubaki leaned in to kiss the love of her life one more time. “So then, Senku?” she whispered against his lips.

Senku’s smile- this genuine and beautiful grin- made her all the more certain. “Let’s get experimenting, Tsubaki.”

And so, much like that one night a lifetime ago, one kiss became two, then three, and so, so much more.


…

…

“Ha…! That was good, oh so good. Was it good for you, Love?”

“....”

“Just one more. You can hang on for another round, can’t you, Senku?”

“...Y-you….”

“Hmm?”

“Y-you… are… fucking INSATIABLE, woman!”

“Oh!”

“NO, I can't hang on for another round! I barely survived the last two!”

“Oh, um… perhaps I went a bit overboard.”

“I can’t feel my legs anymore, Tsubaki! Are they even still there, or did they get severed when you shattered my fucking pelvis?!”

“Teehee! I’m sorry, Love.”

“Haaaaaah!”

…

…

“But… I guess seeing you all happy and shit is worth potential limb loss.”

“I’m glad. And you’ll be glad to know your legs are still intact.

“Oh, joy.”

…

“Happy Birthday, Tsubaki.”

“Thank you, Senku. For this. For everything, really.”

“Hmm.”

“I love you.”

“....”

“...You still can’t say it, huh?”

“...Sorry. Not all four letter words come that easy to me.”

“Hmm.”

“But… you know I do… right?”

Chu!

“I do.”

Notes:

Happy Birthday Shishio Tsukasa, and a very happy birthday to Tsubaki and all other Tsukasa variants out there!

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