Young Roger Court (Richard O'Sullivan) arrives in Venice expecting to meet his father, Maj. Court (Trevor Howard), an influential British diplomat, little realizing that his father has been kidnapped by foreign agents and is being held as the drugged hostage of Dr. Vivaldi (Eduardo Ciannelli). Unable to find his father and distrusting the police, Roger latches on to hotel worker Roberta (Alida Valli) and her friend Joe (Richard Basehart), who are preoccupied with problems of their own.