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.
Ash and Machin are recalled to London from a bomb disposal course and attend to a bomb in a school yard that is dangerously close to a telephone exchange. When another bomb is uncovered close by, Ash reluctantly allows Machin to defuse the his first bomb.
Ash is ordered to take a defused bomb direct to a scientist working on ways to get around the booby-trap timer.
A bomb under a nightclub provides 347 Section with an opportunity for a little recreation. Ash admits that he is smitten with Susan.
Ash and Susan go off for a quiet weekend away but Ash is recalled to deal with an unexploded bomb and Susan returns home to find her husband waiting for her.
Ash works on a bomb outside a destroyed office building while the men of 347 Section try to rescue a woman trapped inside and with another bomb right beside her.
Newly-promoted Major Francis takes command of the unit and proceeds to enforce new disciplines, but his chief target seems to be Ash and his men.
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.
The Germans have been dropping a new butterfly bomb that are proving difficult to defuse. 347 Section is one of a number of units called to a small English village to deal with them.
Ash is assigned to Cambridge to help Dr. Gillespie with the new Y fuse, and meets Susan again.
The Company packs up and moves to the coast where they are to clear the beaches of dozens of British mines laid in preparation for a German invasion.
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.