The book was first published in serialized form in November 1942 in Liberty, with its first bound publication in 1943 in an American edition by The Crime Club.

In Washington, D.C., a young woman whose father has invented a new form of synthetic rubber requests Simon Templar's aid when she receives a threatening note.

Before long, The Saint is drawn into a web of war-related intrigue involving what appear to be gangsters, but soon turns out to be groups with differing opinions as to what it takes to be patriotic.

The book reveals that, instead of enlisting to fight in the war, Templar has instead been working behind the scenes, carrying out quiet missions against enemy agents and, unusually for the character, his efforts in this case are actually supported by law enforcement.

As was the case with the previous book, The Saint Goes West, The Saint Steps In includes in-jokes related to Simon Templar's "second career" as a B-movie hero (as a series of films featuring the character made by RKO Pictures was underway at the time of its publication).