This poem focuses on the eve of St. Agnes, a night when, according to legend, a young maiden can receive a vision of her lover.
At the opening, we see the story from the point of view of the beadle, an old man who is paid to say prayers. He feels the cold of the evening very strongly and is prepared to stay up all night praying for the souls of "sinners."
This opening creates a sense of cold and foreboding that foreshadows that all will not go well on this night. Then we move from the beadle to the big party being held at the castle that evening.
It's important to note that although there is much festivity, the trumpets greeting the guest are "snarling," another indication that all might not be well.
Following the party, the focus moves to one person, Madeline, who looks forward to the night because she hopes for a vision of her lover:
Young virgins might have visions of delight,
And soft adorings from their loves receive
Upon the honey'd middle of the night
Madeline will have more than a dream. Her beloved, Porphyro, has secretly crept into the castle. He is an enemy of Madeline's family, so he is taking a big risk to be at the castle.
Porphyro finds the aged Angela and persuades her to let him hide in a closet in Madeline's room so that he can watch her without her knowing he is there.
Once Madeline is asleep, the moon falls on her and Porphyro sees her looking like a saint or angel in all her glory. He lays out treats for her:
Of candied apple, quince, and plum, and gourd;
With jellies soother than the creamy curd,
And lucent syrops, tinct with cinnamon;
Manna and dates, in argosy transferr'd
From Fez; and spiced dainties, every one,
From silken Samarcand to cedar'd Lebanon
When she wakes up, he persuades her to run away with him so they can marry. She is frightened, but she goes off with him into a storm. The two lovers disappear and are never heard from again. We don't know what happens to them, but the end of the poem suggests the worst: the Baron, her father, dreams of "woe," and his "warrior guests" have nightmares "of witch, and demon, and large coffin-worm." Old Angela dies, and the Beadle sleeps on cold ashes.