Twenty years ago, John Balthazar, a notable and brilliant Cambridge mathematician, left England abruptly as he found himself falling in love with a woman who was not his wife. No one hears from him for 20 years and it's assumed he's dead. He travels to China where he steeps himself in the culture and returns incognito 20 years later with his Chinese pupil, Quong Ho. They live in a remote farmhouse where he stays in blissful ignorance of the events of the First World War until a German zeppelin crashes nearby and blows up his house. Abruptly brought back to the reality of life in 1916, Baltazar finds that his wife has long since died but has left a son he did not know existed, who has coincidentally met his former love in a nursing home. The second half of the book picks up the story from there.