A marsh wakes up at dawn, and the poem watches it happen. The first couplet sets the scene: a fen, all slush and mud, still asleep until daybreak stirs it. Then thin wisps of smoke, or mist, begin to rise and settle over everything like a damp cloak. The image is soft and quiet, almost tender, as if the land is being tucked in rather than uncovered.
The last couplet changes the tone. The mist is "woefully fatal as wondrous fair," so it is both lovely and deadly. The speaker turns to "you" and says that in the fog's embrace your sight is impaired. Being held by the fog sounds comforting, but it also means losing the ability to see where you are going. The speaker seems to feel a mix of awe and unease. The mood is hushed, dreamy and a little threatening, the kind of beauty that makes you slow down and also makes you wary. The poem never says what the danger is, and that vagueness adds to the sense of being lost.