Captain Mainwaring gives a 'top secret lecture on the coastal defense warning system his platoon will guard for a weekend, only to learn even the warden knows. Trouble with Jones' van makes them late to relieve regular troops; they set up camp and have porridge in their bumbling fashion. After a nightly air raid, the vicar comes to say mass. Only then do they see a German bomb, caught in the very wires they guard. Getting it down is a job the GPO turns down, but Mainwaring and other old-timers volunteer themselves, all failing, while wisely reluctant Frank is forced to try more unwise monkey-methods.