The speaker opens the door one bright morning and finds no one there, only a single red rose. It looks "lonely and alone," and the speaker brings it inside, curious about who left it. The middle of the poem is a string of comparisons: the rose is dark red, red as blood, deep as the ocean, gentle as touch, pure as love. These images move from intense and physical to soft and tender, so the rose starts to feel like more than a flower. It becomes a symbol of love itself.
By the end, the speaker lets go of the mystery. Who left it doesn't matter; what matters is the love the gift represents. There is also a hopeful, slightly wistful note. The speaker imagines a day when they will meet "that someone" standing at the same door, on the same bright morning. The mood is warm, gentle and quietly romantic, with a little loneliness underneath that the rose seems to ease.