Dr Alison Howden has devoted her life to understanding the base rules of the universe, the ones that hold even in multiple universes. If she can just understand how everything works then she maybe will find it easier to live by her one rule: never go back on a promise. Especially her marriage vows. Then Jack explodes into her life. He reveals that all her theories are true - he's from another universe. But his world is crumbling. He can save them but he's trapped in Alison's universe and only she can help. Delighted to be needed for once, and keen to see the multiverse for herself, she agrees to go. Jack promises he'll bring her back home. Or perhaps to a more perfect versigon of home - one where keeping her promises isn't quite so hard. All she has to do is hold her nerve, trust him and take the leap.