During a coroner’s inquest, Swallow questions Peter Grimes about the death of his apprentice at sea. Grimes testifies that the boy died when they ran out of drinking water. Though Swallow rules the death accidental, he warns Grimes not to take on another apprentice unless he provides a woman to care for him. The fisherman replies that this is his intent. When the hall empties, Ellen Orford promises to help Grimes.

Balstrode spies a storm and from the harbor, Grimes calls for help to land his boat, but only Balstrode and Ned Keene respond. Keene tells Grimes he has found a new apprentice for him. Balstrode tries to convince Grimes to join the merchant marine, but he wants to earn enough money to get married to Ellen. That night, at the storm’s height, Auntie unwillingly admits a group of fishermen and other townspeople to her tavern, The Boar. Grimes enters, declaiming a poetic fantasy that mystifies the onlookers. Hobson and Ellen arrive with John, the new apprentice, and Grimes takes the boy back out into the storm to his hut, over angry protests.

Ellen is concerned about the new apprentice’s torn clothes and bruised neck. She asks Grimes to let the boy have some rest, but he strikes her. Auntie, Keene, and Bob Boles see the incident and inform the townspeople. Despite Ellen’s protests, Boles rallies a mob and leads it off to apprehend Grimes. At his hut, Grimes orders John to dress for work. Raving to himself, Grimes imagines making enough money to marry Ellen, but his vision turns to the dead apprentice. As the mob is heard approaching, Grimes rushes John out the back door and, in his haste, the boy falls over the steep cliff. Grimes escapes from the mob and the villagers depart.

A few nights later, a dance is under way at Moot Hall. Mrs. Sedley tries to interest Keene in a theory that Grimes has murdered his apprentice. Balstrode enters with Ellen, who is carrying the apprentice’s wet jersey. When they leave, Mrs. Sedley calls for Swallow, who orders Grimes’s arrest. Several hours later, Grimes staggers in, exhausted and raving. Balstrode tells him to sail out of the harbor and sink his boat. As dawn breaks, the villagers begin their daily chores, while the coastguard reports a sinking vessel out at sea.