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The baby, Kate, is his and she isn't. His Kate never cried. Not once. He and his soon to be ex wife had waited and waited, but this Kate is alive and thriving, she cries the second he puts her down. This Kate was born on a parallel world, the Doctor saved her from it before it was destroyed. He brought her back, like a fairy child. A changeling child like from the myths.
Alistair doesn't know if he should thank the Doctor or throttle him for the gift of her. He cannot tell his soon to be ex wife about her existence, couldn't explain that here is our daughter from another world. Even if she believed him, he wasn't sure it would be enough to save their marriage. Briefly he'd considered giving Kate up, to let her have a proper family. But then he held her in his arms and she'd looked up at like he was everything, the thought vanished from his mind.
“You know you are going to have to put her down sometime,” Liz whispered, careful not to wake Kate up, smiling at seeing the facade of the soldier slip for once.
“Try telling her that. She screams blue murder when I do,” Alistair tried not to smile, he did. But when she's crying he knows she's alive, when she's in his arms he knows she's breathing.