Harry Palmer has left the British Secret Service and become a private detective. One of his first assignments is to deliver an apparently innocent thermos flask to an old friend in Helsinki. Palmer is suspicious of the flask and his friend's motives.

A plain, middle-aged married man, Richard Sherman sends his wife Helen and his young son Ricky off to vacation at the seashore in Maine during a sultry summer in New York City. They escape but he must remain behind in the heat, to make money. He sees them off at the train station, promising to eat properly, and neither smoke nor drink. He will be sensible, not play around as soon as his wife leaves like so many other men. That is until he meets his...