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September 1940: Brian Ash, a direct entry officer of the Royal Engineers after ten weeks of basic training at the Hampshire Depot, joins 97 Tunnelling Company. Without benefit of training as a bomb disposal officer, he is immediately put in charge of a section whose previous officer was killed by a bomb. The unexploded bombs they deal with have been armed by electrical fuses that activate after being dropped, but sometimes fail to go off on impact. If they fail to explode on impact, they can detonate when vibrations caused by movement or contact close the circuit. Under the tutelage of Sergeant James, he defuses a relatively straightforward type 15 fused bomb, but is dismayed to learn that the Germans have deployed an even deadlier fuse, the type 17, that is rigged with a clockwork long-delay detonator activated by the bomb's impact.

Ash is reprimanded, first by Francis for a newspaper story about the section that the 2IC believes injures the dignity of the service, then by Luckhurst for his reckless behaviour when defuzing a low-priority bomb. Ash becomes acquainted with new officer Ken Machin, who is assigned to his tutelage and billeted with him. Ash and Machin find they are to be sent on a hastily developed new disposal course.

On their course, Ash and Machin learn that the boffins have developed a powerful electromagnetic 'clockstopper' to counter the type 17's fuse. Ash gives in to Machin's pleadings, allowing him to finish disarming the type 17 discovered in a residential garden while Ash deals with a Category A bomb near a telephone exchange. In Ash's absence, the type 17 explodes, killing Machin, despite the clockstopper. Ash later finds the Category A bomb has ruptured without exploding, and has an intact type 17 fuse, which is also rigged with an anti-handling device called the Zeus 40 that detonates if there is an attempt to extract the fuse.

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.