When You Disappeared opens with Simon Nicholson vanishing on the morning of his fortieth birthday, leaving his wife Catherine and their children with no note, no body, and no answers. The novel follows the years after his disappearance as Catherine struggles to rebuild a life for her family while never fully closing the door on what happened to him.
Seven years later, Simon returns — alive, largely unchanged, and reluctant to explain where he has been or why he left. John Marrs structures the book around alternating timelines and shifting points of view, gradually revealing the events leading up to Simon's disappearance and the truth he's been concealing, which turns out to be far more unsettling than a simple case of a man walking out on his family.
As a domestic thriller, the novel is less interested in a whodunit mechanism than in dismantling the assumption that a marriage — even one built on genuine love — can be fully known by either partner, and in tracking how each character's version of events differs from the others'.