The series chronicles the exploits of the fictional Royal Engineer Tunnelling Company 97, which has been made a bomb disposal unit to deal with the thousands of unexploded bombs ("UXBs") in London during the Battle of Britain.

Brian Ash is a young lieutenant who is assigned to a UXB unit in the early days of World War II. UXB (UneXploded Bomb) is the signal that an aerial bomb has not exploded. Ash's job is to deactivate German bombs, some of which have fuses specifically designed to kill him.—Anonymous