Seer Blood runs in the Weasley Family.
After the war is over and Voldemort has finally been destroyed, Ron and Hermione look around and try to find a victory amidst all the death that has plagued their families and friends, but there is none to be had. When Ron reveals to Hermione that Seer Blood has always run in his family, it gives them a chance to fix things, by letting Ron go back in time and change the course of the war.
But things are thrown off from the start, because simply viewing something alters its existence, and Ron was planning to arrive in third year, not first. Now he's living his life over, except no one is the same. He misses his best friend and his wife, but the things he has to do to keep them safe may change what they are to him in the future.
But it's worth it for this second chance. If he has to give them up, it's a small price to pay to keep them all alive.
But even that might prove too difficult.
Book One: Sense of the Soul
