Coleridge's poem is an example of a frame tale, a story within a story. As it begins, the ancient mariner stops one of three wedding guests on their way to the ceremony. He begins to tell the stranger his story. The old man's appearance and bearing are so compelling that the Wedding Guest [sic] cannot turn away. He sits down to listen.
The remainder of Part I consists of an introduction to the mariner's fantastic tale. As he speaks, the Wedding Guest grows restless but cannot stop listening, even though the wedding he was attending has now begun. This detail emphasizes the compelling nature of the story in progress.
The mariner's story relates details of the terrors the ship encounters as it is driven toward the South Pole and trapped in the ice, which "cracked and growled, and roared and howled." At this point, the albatross enters the story to save the ship and accompany the sailors on their way. The bird comes to their call, "for food or play." It remains faithful to them as the ship enjoys "a good south wind." The albatross has brought them good luck and companionship.
At this point, the Wedding Guest breaks in when he notices the mariner's sudden anguished expression: "God save thee, ancient Mariner! / From the fiends, that plague thee thus!--Why look'st thou so?" Part I concludes with the mariner's shocking confession: He had shot the albatross with his crossbow. With this admission, it becomes clear that the remainder of his story will be very dramatic.