 Private detective Edward Mercer (Richard Todd) goes to Venice at the request of a French insurance company to locate a brave Italian whom they wish to reward for his part in the rescue of an Allied airman shot down during the war. At least, that is what Mercer thinks as he steps off the steamer at the Piazzo San Marco and is greeted by a smiling street photographer, Cassana (John Gregson). Mercer makes his way to a shop and finds his first contact dead from a knife stab, and the trail leads him to Adrianna (Eva Bartok). He faces danger from police chief Spaloni (George Coulouris) and also from a group of foreign patriots, led by Count Borian (Walter Rilla) and Lieutenant Longo (John Riley), who want to use him as a stool-pigeon for a planned Coup d Etat. A hectic race across the roof tops, high above the great square, brings Mercer to grips against his unknown enemy.