Three guys are on the way to a wedding celebration when an old sailor (the Mariner) stops one of them at the door (we'll call him the Wedding Guest). Using his hypnotic eyes to hold the attention of the Wedding Guest, he starts telling a story about a disastrous journey he took. The Wedding Guest really wants to go party, but he can't pry himself away from this grizzled old mariner. The Mariner begins his story. They left port, and the ship sailed down near Antarctica to get away from a bad storm, but then they get caught in a dangerous, foggy ice field. An albatross shows up to steer them through the fog and provide good winds, but then the Mariner decides to shoot it. Oops.
Pretty soon the sailors lose their wind, and it gets really hot. They run out of water, and everyone blames the Mariner. The ship seems to be haunted by a bad spirit, and weird stuff starts appearing, like slimy creatures that walk on the ocean. The Mariner's crewmates decide to hang the dead albatross around his neck to remind him of his error.
Everyone is literally dying of thirst. The Mariner sees another ship's sail at a distance. He wants to yell out, but his mouth is too dry, so he sucks some of his own blood to moisten his lips. He's like, "A ship! We're saved." Sadly, the ship is a ghost ship piloted by two spirits, Death and Life-in-Death, who have to be the last people you'd want to meet on a journey. Everyone on the Mariner's ship dies.
The wedding guest realizes, "Ah! You're a ghost!" But the Mariner says, "Well, actually, I was the only one who didn't die." He continues his story: he's on a boat with a lot of dead bodies, surrounded by an ocean full of slimy things. Worse, these slimy things are nasty water snakes. But the Mariner escapes his curse by unconsciously blessing the hideous snakes, and the albatross drops off his neck into the ocean.
The Mariner falls into a sweet sleep, and it finally rains when he wakes up. A storm strikes up in the distance, and all the dead sailors rise like zombies to pilot the ship. The sailors don't actually come back to life. Instead, angels fill their bodies, and another supernatural spirit under the ocean seems to push the boat. The Mariner faints and hears two voices talking about how he killed the albatross and still has more penance to do. These two mysterious voices explain how the ship is moving.
After a speedy journey, the ship ends up back in port again. The Mariner sees angels standing next to the bodies of all his crewmates. Then a rescue boat shows up to take him back to shore. The Mariner is happy that a guy called "the hermit" is on the rescue boat. The hermit is in a good mood. All of a sudden there's a loud noise, and the Mariner's ship sinks. The hermit's boat picks up the Mariner.
When they get on shore, the Mariner is desperate to tell his story to the hermit. He feels a terrible pain until the story had been told.
In fact, the Mariner says that he still has the same painful need to tell his story, which is why he stopped the Wedding Guest on this occasion. Wrapping up, the Mariner tells the Wedding Guest that he needs to learn how to say his prayers and love other people and things. Then the Mariner leaves, and the Wedding Guest no longer wants to enter the wedding. He goes home and wakes up the next day, as the famous last lines go, "a sadder and a wiser man."