Cambridge, 1940. Snow is falling thick and fast on a cold winter's night. As a college porter makes his nightly rounds, he is startled to hear a boy's desperate screams for help coming from the icy river below. But by dawn the river has claimed its victim.

When the following night an Irish Republican slogan is left at the scene of a factory explosion, Detective Inspector Eden Brooke questions whether there could be a connection between the two events. As more riddles come to light, he begins to close in on a killer, but there is one last twist: it seems that the boy had his own startling secret.