Newly-promoted Second Lieutentant Brian Ash joins his new regiment only to find that they are assigned to bomb disposal in London. Ash is thrown in at the deep end when he and his men are sent to a bomb site.

Ash tackles a bomb that is not considered vital with a new fuse extractor that doesn't work properly. New Lieutenant Machin is placed with Ash to get some on-the-job training. Major Francis criticises Ash after a patriotic magazine article profiles 347 Section.

A bomb under a nightclub provides 347 Section with an opportunity for a little recreation. Ash admits that he is smitten with Susan.

347 Section cooperates with the Royal Navy to deal with a bomb that has a 17-second fuse. Ash decides to ask Susan to marry him.

Ash convalesces after a near-fatal accident while defusing a mine. With his mobility severely impaired, he is to be reassigned to light duties away from the bomb disposal unit.