Walter is a young Australian who comes to England (falling in love with a beautiful and feisty schoolteacher on the boat journey) to follow his dream of being a scientist at the Cavendish laboratory in Cambridge. There he falls under the spell of the charismatic scientists involved in splitting the atom and the other experiments that eventually led to the building of the atomic bomb. In 1945, in London, an older, disillusioned Walter struggles to keep his marriage together and to come to terms with what he - as a pacifist - saw as a terrible betrayal by his mentors and his best friend Alan Nunn May. Meanwhile, Nunn May is back in London, keeping a low profile and being shadowed by a rather inept and unenthusiastic spy. Walter is the only person Alan has seen since his return and so unwittingly, he and his family fall under surveillance too. The story that unfolds is an engrossing tale of friendship and betrayal, of forgiveness and enduring love and of the dangerous burden of scientific knowledge.