A writer named Adrian Messenger (John Merivale) believes a series of apparently unrelated "accidental" deaths are actually linked murders. He asks his friend Anthony Gethryn (George C. Scott), recently retired from MI5, to help clear up the mystery. However, Messenger's plane is bombed while he is en route to collect evidence to confirm his suspicions and, with his dying breath, he tries to tell a fellow passenger the key to the mystery.