Baxter, a bull terrier, is taken from the pens and given to an old woman. Baxter hates the old woman’s bland lifestyle and reacts to her habits with disgust. In contrast, he becomes obsessed with the young couple across the street as he observes their nightly lovemaking sessions. Baxter attempts to communicate his dominance over the old woman by causing her to stumble, but his plan backfires. The old woman’s condition deteriorates, and ultimately Baxter kills her in order to be adopted by the young couple.
Baxter enjoys his life with the young couple, splitting his time between the sensual woman and the earthy man. He brings them dead animals in an attempt to show them who he is. His idyll is broken when the couple has a baby and begins to neglect him. Baxter hates the weak and helpless child. He attempts to kill it, but his plans again backfire. Ignorant of Baxter’s murderous intentions, the couple gives Baxter to a neighborhood boy.
Baxter thrills under the firm hand of his new master, a budding sociopath and Hitler fanatic. Baxter kills a stray dog to show the boy who he is, and Baxter believes that they come to a mutual understanding. The boy begins to see a girl from his school who reminds him of Eva Braun. Baxter impregnates the girl’s spaniel, though his own sexuality disgusts him. When the boy commands Baxter to kill a classmate, Baxter refuses and realizes that the boy doesn't understand him after all.
The girl’s spaniel gives birth to puppies, and Baxter reacts to them with mixed emotions. In an attempt to emulate the final days of Hitler, the boy kills the puppies. In reaction, Baxter decides that the boy must die. The boy attacks first, but Baxter manages to gain the upper hand. When the boy commands him to heel, Baxter finds that he cannot disobey, allowing the boy to kill him. Later, the boy breaks into the old lady’s abandoned house and observes the young couple across the street. In a monologue echoing Baxter’s, the boy plans to kill his parents and be adopted by the couple.