Preface - the narrator (Elliot's father) looks back on his life. What do we sacrifice for love? We can't choose. Harriet was 32, a policewoman, when she met Ben. She was lonely, mailing to herself daily. The narrator admits that he has access to the mail, that this book isn't a product of his imagination. Elliot was troubled, according to the social worker.
Harriet lived in a flat in Stoke. She had been dismissed 6 weeks before her story starts. Ben had harshly broken up with her a while before. She had loved him at first sight and thought the feeling was mutual. They had a love of poetry in common. Her work partner had been Sabi Khan.
She finds a withdrawn library book in a 2nd hand shop - a self-improvement book which mentions that we long for the familiar even if it's a bad pattern we've fallen into. It also mentions that sexual attraction weakens rationality because procreation is an instinct. A "he's going to kill me" message is written in it. She goes to the library and finds that it was last taken out by Beth Asher, who'd died of cancer a year before. Her husband David died later - an accident on a cliff, maybe suicide - the body never found. Harriet goes to their cottage and finds Ben there, looking after Elliot who's upstairs. Elliot resembles Ben. Had Ben been having an affair with Beth?
She discovers that Beth's body disappeared from the hospital the night she died. She, David and Ben all worked at the same physics lab from which some Cobalt 60 had been stolen. The lab boss asks Harriet to check Ben's cottage with a radiation detector. She finds a poem for her in the toilet. Elliot says that Ben often talks about her. His car sets the detector going. Harriet hopes that solving the case will help her get her job back. But Sabi makes the investigation official. When Sabi and she investigate they follow Ben and Elliot. Elliot gets away. During a fight, Ben pushes Sabi into a ravine.
Elliot tells Harriet he knows a secret. Ben reminds him not to tell anyone. Ben gives himself in, first passing Elliot to the housekeeper, Cynthia. In the courtroom Elliot shouts that Ben's innocent, that Harriet pushed Sabi over the edge.
Sabi tried to deal with a minor criminal, Monroe, who drew a knife and slashed at Sabi. Harriet hit Monroe. They fought. Monroe ran into the path of a train and died. An investigation suggested that she used unnecessary force and caused Monroe's death, hence her dismissal. Later she was sent a video showing her innocence.
Part 2. Elliot's a student at Lincoln college, Oxford. 2 profs talk to him, saying that he was wasting his time. He seemed very self-contained. Prof Hoight got a Nobel using formulae that Elliot had jotted while she was disciplining him. Next day Elliot disappeared. Dead?
Thanks to the video evidence, Harriet got her job back and her boss took early retirement. She replaced him. When Ben's released, Elliot goes to her, promising to tell her the secret if she came with him. She does so, but suddenly fears she's being led into a trap.
She's shown an underground lab. A screen there shows Ben. Ben had discovered a time travel device (cobalt 60 being an ingredient) and had taken his father back to the moment of Beth's death. Ben rushed them all 200 years into the future. Beth recovers and lives with David, having 3 kids, but Ben returns to look after Elliot so that Elliot (who is the younger Ben) can discover time travel. Ben/Elliot and Harriet go back to 1989 and live happily out of the way of paradoxes. Various loose ends are tidied up - it's easy to see now why Ben kept talking about fate and inevitability.