Cary Grant is "Mr. Lucky," a self-serving swindler named Joe who takes the identity of a dying man who's 4F so that he won't have to go into the service. The name he takes is that of a Greek man, Joe Bascopoulos, but the name will bring him more than just a 4F classification.
Needing money to get his gambling ship started, he sees an easy mark in Dorothy Bryant (Day), one of the administrators of a War Relief Fund. He uses his considerable charm to persuade her to let him run a casino night as part of the organization's big fundraiser. The plan is to use cash boxes with false bottoms and take off with nearly all of the money. Just one hitch - Joe has fallen in love with Dorothy.