The Goodies are annoyed when they discover that the Army is building a military establishment on what had been supposed to be a children's playground.
Disguised as members of the military, they provide the plans to the soldiers who are to build the equipment. The Goodies "assist" the Army and their plans lead to the Army 'equipment' looking more like a children's playground than the sleek military equipment the Army is expecting.
Then the Army gets another shock — the people who the Goodies had arranged to turn up to test the equipment, are not exactly what the Army had been expecting to see.
