The episode opens with a General briefing two others in the War Office building in 1941: the General briefs the others about a weapon called the High Explosive Attack Device Propelled by Ultra High Frequency (HEADPUHF) and the test called "Operation Catherine Wheel".
Several local Home Guard units will be roped in to help during the test and the smarmy Captain Stewart tells them he will get the Walmington-On-Sea brigade to do the dirty work.
Mainwaring debriefs the platoon down in the church crypt to maintain secrecy, but ARP Warden Hodges barges in.
However, the men are annoyed to find that their 'special duties' involve dirty work such as peeling potatoes and digging latrines, much to Hodges' amusement.
It rolls out of the base during its demonstration test and the Walmington platoon find themselves having to pursue it – if they don't stop it, it will explode and the town will be destroyed.