An FBI agent works with a refugee scientist and the Coast Guard to crack a Soviet spy ring in Boston.

Burglar Nat Harbin and his two associates set their sights on wealthy spiritualist Sister Sarah, who has inherited a fortune -- including a renowned emerald necklace -- from a Philadelphia financier. Using Nat's female ward, Gladden, to pose as an admirer and case the mansion where the woman lives, they set up a perfect break-in. Things get complicated afterwards.