What is the plot of the poem "The Lady of Shalott"?

The central conflict of the story told in Alfred, Lord Tennyson's "Lady of Shalott" is that the main character is bound by a curse that requires her to weave steadily by night and day, never stopping to so much as look at the world that passes by her tower. Interestingly, she does not know what the curse may be, so the question that pervades the poem is whether she will acquiesce to the curse or whether she will take a chance and defy her fate. 

Part 1 of the poem introduces the setting and the main character, the Lady in the tower who people hear singing but never see. Part 2 is the rising action. The curse is explained; then we learn that the Lady is generally content to watch the "shadows of the world appear" in a mirror she has hung behind her loom to reflect the passersby. She sees a variety of people pass, including young boys and market girls, an abbot, a shepherd, and a page, and she even sees a funeral procession. These sights don't particularly move her, but the rising action begins to move toward its peak when she sees "two young lovers lately wed." This moves her to discontent at the end of Part 2.

Part 3 brings the rising action up to its climax. "Bold Sir Lancelot" appears in her mirror as he comes riding by. He is glittery, handsome, and sings "tirra lirra." The Lady is deeply moved by his appearance and song, and "she left her web, she left her loom, she made three paces through the room." This is the high point of the action, resulting in the breaking of the loom and cracking of the mirror, signifying that the curse has come upon her.

Part 4 is the falling action, or denouement, and resolution. The Lady leaves her tower and comes down to the river. She finds a boat, paints her name on the prow, and gets in. As the weather becomes stormy, she lies down in the boat and floats toward Camelot. She sings as the river bears her "far away," and "singing in her song she died." The boat carrying her corpse floats into Camelot where the residents all come to examine the craft from which they have heard "a carol, mournful, holy" arising. They are full of questions about her although they do know her identity because of the writing on the boat. Finally, Sir Lancelot views the body. He reciprocates the interest she had in him, although she will never know that, and he invokes God's blessing on her. 

Thus the plot is that the Lady lives under the mysterious curse that is upon her but defies it when she sees Sir Lancelot, whereupon the curse proves fatal to her, resulting in her corpse arriving in Camelot where Sir Lancelot expresses interest and pronounces a blessing over her.