1920. Lord Fountain discovers that Maggie, his working-class mistress, has deserted him for Towser Griddle, an eccentric artist who keeps her in a cage while he paints her. She has left behind the couple's recently delivered twins: Bernard, a fat baby, and Timothy, who is thin.

As time passes, Fountain grows to dislike his sons. At the age of eleven, they fall in love with the bailiff's daughter. Fountain kills her under the pretence of a hunting accident. Bernard and Timothy are distraught.

Years pass. Fountain, desperate to produce a legitimate heir, marries several times, but no child is born. The twins are sent Oxford, where they prove socially ineffectual, and develop a pathological fear of Hitler as the clouds of war gather. Meanwhile their father entertains high-ranking Nazi General Remnitz.

When war breaks out, Fountain secures his sons commissions in the army. Unwilling for them to fight against Germany, where he has industrial interests, Fountain has them posted to the Far East. During a retreat the two get lost in the jungle, where they befriend the animals and a Japanese soldier called Ishaki. They live out the war happily in the jungle, singing and playing the flute.

After the A-Bombing of Japan, Bernard and Timothy return to England where, at their father's behest, they go to work for a merchant bank. They hate it and desert their jobs. Having married his seventh wife but still not produced an heir, Fountain is assured by an eminent geriatrician that he is not sterile.

The twins become zookeepers and share rooms in London. They also have girlfriends, Laura and Poppy, but their relationships are strained. The brothers feel they are not cut out for loving and suffer impotence. After their father dabbles disastrously in manufacturing dog food, the twins release all the animals from the zoo. They spend time in prison as a result.

On release, Timothy sees Laura while Bernard visits captive dolphins. Timothy is concerned about a hurt he feels inside, but this just aggravates Laura and her belief that he does not love her. Laura and Poppy catch up with Fountain and threaten to sue his sons. The three conclude that they hate the twins and write to the pair. On reading the letter, Bernard falls into a coma, dreaming of porpoises. When he begins to recover, he calls for his mother, who is brought across London in her cage.

Fountain dreams an interview with God. He proves amiable to the belligerent old man, agreeing with his views on the need for traditional hierarchy.

Evicted by their father, who has bought their house, the twins go to live in a derelict swimming baths, candle-lit and full of rubber animals. They swing on trapezes and sing rhymes. They are visited by Laura and Poppy, who have become bitter at the twins' failure to offer them marriage, domesticity and riches, and their abdication of responsibility. They reveal that Fountain is buying the swimming baths to turn into a supermarket. The twins invite the girls to live with them, but they dismiss the brothers, Poppy suggesting they shoot themselves. When she leaves, they play Russian Roulette, but succeed only in bursting one of the inflatable animals.

Fountain throws a dinner party on the pretext of the twins' birthday. Maggie, Griddle, Laura, Poppy, Ishaki, the boys' old nanny and the zoo director are all present. General Remnitz, now Public Prosecutor having had his Nuremberg conviction conveniently quashed, is unable to attend. The guests are called on to denounce the twins, and Fountain gives them their birthday presents: a pair of life-sized doll princesses. The twins leave to the derisive laughter of the guests. Maggie, hearing a voce in her head, draws a gun and shoots Fountain dead. The new Lady Fountain reveals that her husband has left everything to his sons in his will.

Maggie is hanged. The twins visit their father's house and inspect the portraits of their ancestors. Griddle and the police break into the swimming baths, only to find them deserted.

Meanwhile, Bernard, Timothy and Ishaki paddle happily up a tributary of the Amazon.