Farley Granger plays a casually larcenous New York City mailman who steals a shipment of money. Granger's excitement over this windfall turns to terror when he discovers that the money was part of a transaction between gangsters. Harassed by both crooks and cops, Granger lives to regret his impulsive theft--especially when it is tied in with a murder. The story is wrapped up in spectacular fashion with a climactic car chase. Farley Granger's costar in Side Street is Cathy O'Donnell; both were on loan to MGM from Samuel Goldwyn, and both were banking on their previous successful teaming in RKO's They Live By Night.