As the poem begins, frost starts creeping through the midnight. Coleridge—writing from his own point of view and starring as his own speaker—stays up alone, hosting a pajama party of one. All the people living in Coleridge's cottage are asleep, and his baby son slumbers in a cradle next to where Coleridge is thinking. He meditates on the intense stillness and silence of the night, which starts to seem kind of disturbing. He imagines the secret goings-on that must be happening in Nature and in the town.
A film of soot flutters on the grate of the fireplace—a phenomenon known as a "stranger." Coleridge thinks it's similar to him, since he's the only person awake in the house and his flickering thoughts are kind of like this fluttering "stranger." It reminds him of how the Spirit (either the human spirit or the Holy Spirit or both) searches for an echo of itself in the world of thought. He starts thinking about when he was a kid in school, day-dreaming and looking at a "stranger" moving on the schoolroom's fireplace, imagining that some real stranger (actually, one of his beloved relatives or friends or someone otherwise familiar to him) would suddenly show up and make things cheerier and less boring. Church bells always made him feel this way too.
Finally, Coleridge starts to think more about his baby son. He hopes that the baby (Hartley Coleridge, by name) will grow up in tune with Nature—unlike Coleridge himself, since he went to school in London. This will be beneficial, since Coleridge believes that God communicates with human beings directly through Nature. It's a kind of "eternal language" that teaches people about the creator's existence. The poem ends with Coleridge predicting that his son, by experiencing God at all times of the year, will be able to find happiness no matter what the season—including winter, when the frost hangs up icicles that reflect moonlight back to the moon.