Al Goddard (Alan Ladd) is a detective employed by the U.S. Postal Service to investigate a murder of another detective. Once Goddard arrives in Indiana, he locates the key witness, Sister Augustine (Phyllis Calvert), a beautiful nun. Then he infiltrates the murderer's group by convincing the boss, Earl Boettiger (Paul Stewart), that he can be a useful thug. Goddard quickly learns of Boettiger's master plan, but, when the gang figures out his cover, he must save himself and Sister Augustine.