The story which the Ancient Mariner told the wedding guest was 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 and 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 mistake.
Moreover, 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 steered 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. He was 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.