Tony Quinn has invented a bomb that fits inside a fountain pen, and for purposes of the plot, has devised it so that when the cap is off, the sound of Big Ben tolling the hour will set it off. Then he has lost the suitcase containing it. Henry Fowler and Patti Morgan have found the case and pawned it. Quinn tracks them down and offers them thirty pounds for its return.
That's the start, and of course, it becomes much more complicated, involving Sidney Tafler. He owns the club Fowler and Miss Morgan work at, and it soon becomes clear that he's engaged in some shady operations. While the first three run around trying to find the missing pen, Tafler becomes suspicious of what they are up to, and marshals his forces against them.
It's a slow-moving thriller; no one is particularly admirable or terribly interesting, and director Henry Cass seems more interested in making this rather complicated plot come out neatly in seventy minutes than in filling in the details interestingly, although Fowler shows a terror of bells that, with a more interesting performer, might be amusing. The result is a workmanlike second feature.