Frost describes a household habit of locking the house at night while the flowers stay outside, cut off from the light indoors. The speaker then recalls a dream in which someone—or something—tries the door, and the flowers are imagined out there among thieves. Nothing is stolen or harmed, but the next morning a single nasturtium is found on the steps with its bitten stem.
The speaker’s tone is quiet, uneasy, and a little guilty. He suspects he himself may have damaged the flower while sitting outside at dusk, yet the dream makes the outside world feel more shadowy and uncertain. The poem creates a hushed mood, mixing ordinary domestic routine with a faint worry about what the dark might do to the things left outside.