The Vietnam war would had ended by this point. It does not stop The Goodies critiquing American foreign policy.
The Goodies end up at a US air base. Major Cheeseburger (John Bluthal) wants a quiet chat with them and promises them that nothing is getting recorded. Only it is in all his hidden tape recorders and the Goodies know this.
The Americans have invented a nerve gas disguised as tomato soup. The Major wants them to get rid of the soup. The soup is green and taste horrible, so the Goodies flog it to a motorway service station and try to eat the rest.
Only when the Goodies taste the soup, it has a side effect. They get turned into clowns. Soon the whole country falls for some slapstick. Even Harold Wilson gives an emergency Prime Ministerial statement dressed as clown.
It is all a cunning strategy for the Americans to invade Britain and turn it into the 52nd State. The Goodies are not clowning about in order to repel the invasion.
There is a lot of clown type slapstick in this story. It also makes political points lampooning the USA.