In 1965, at the age of 25, Alan Ackland is sentenced to life imprisonment for the murder of a business associate. In 1971, Sylvia Barker, lonely and depressed after a failed marriage and with two young children to bring up alone, seeks a new direction in her life and applies to become a voluntary prison visitor. Several years later their paths cross.

In the near future, a now-elderly Bernard Quatermass investigates the disappearance of his granddaughter and a mysterious cult.