In Boston, a Communist spy ring is after top secret plans of a defense contractor. A scientist employed by the company has stolen part of the plans for $5,000. But he attempts to blackmail the spy ring's leader for another $5,000 to supply the key plans of the project. Luther Shawn, the ring leader, calls his bluff and has his Rottweiler attack, nearly killing the scientist. The spy ring's No. 2 man now attempts to seduce a lonely administrative assistant who has access to the plans. The FBI, led by Erskine and Colby, is assigned to the case after messages from a Communist ship are intercepted. But the bureau's information is only fragmentary. The question is whether Erskine can catch up to the fast-moving spy ring.