At the beginning of the poem, the speaker describes the woman he loves. He says that she is as "fresh as a rose in June." This simile suggests that this woman is very beautiful. The speaker, seemingly prompted by this beauty, then decides to journey to this woman's cottage. He tells us that he sets out during the night, "beneath an evening-moon," the moonlight here adding to the romantic mood of the poem.
The speaker then describes his eagerness to get to this woman's cottage. He says, "With quickening pace my horse drew nigh / Those paths so dear to me." The "quickening" pace implies the speaker's desperate, urgent love for the woman. The fact that the "paths" close to her cottage are "so dear" to him emphasizes how much he loves her.
The speaker then describes how he falls into "one of those sweet dreams" that lovers have. This dream is mostly likely a daydream, because the speaker all the while keeps his eyes on "the descending moon." The moon in this poem is a recurring motif and reflects the speaker's feelings. As the moon sinks lower in the sky, the speaker seems to become more and more anxious. The implication is that he is anxious because he misses the women he loves and doesn't like being away from her. He is anxious to arrive at the cottage and see her once more.
At the end of the poem, the speaker approaches the woman's cottage and as he does he sees that the "bright moon [has] dropped" further in the sky and "behind the cottage roof." The disappearance of the moon seems to then prompt the speaker to entertain a rather somber thought. He thinks to himself, "O mercy! ... If Lucy should be dead." The moon dropping behind Lucy's cottage seems to have left the speaker in a literal and metaphorical darkness.
The speaker's thought, that the woman he has been riding through the night to see might be dead, is the "strange fit of passion" alluded to in the poem's title. It is strange because it is a very dark and somber thought which is born out of a tremendous love.