The Quigleys and the Drysdales are caught in an eternal quadrangle. When Clive and Sonia discover that their respective partners are having an affair, they realize that they are reduced to being the other half of an unpleasant equation. Unwillingly, they join forces in an attempt to save their marriages.
As a result of her affair with Dave, Melissa takes up singing to express her newfound passion - so her husband Clive counters by buying a banjo. While the adulterous couple is being followed by Purvis, the private investigator hired by Sonia, Clive himself is being followed by Lester Whales, the jealous husband of his secretary.
Purvis is following Melissa and Lester is following Clive. Sonia and Clive are following Dave and Melissa and then Clive hits the jackpot unexpectedly.
All roads lead to Sonia's country cottage but Dave and Melissa's car breaks down en route, due to a bit of petrol-tank sabotage courtesy of Clive and Sonia. The latter couple do make it to the cottage only to discover another pair making full use of the facilities.
Sonia and Clive are caught by the police lurking with intent in the undergrowth of Sonia's garden. Meanwhile, their respective partners, Melissa and Dave, find that their prospective tryst in a country love nest is well and truly spiked.
