The poem consists of dialogue between two characters, a husband named Fred and a wife whose name is never given. The premise of the dialogue is revealed in the opening eight lines: The two have gone for a walk along a stream, named the West-running Brook, which is an actual stream just outside the town of Derry, New Hampshire, where Frost and his family lived for a time.

The poem shifts from wife to husband and then back and forth save for a curious and much-debated passage (Lines 24-31) set off by parentheses in which apparently neither character speaks. The speaker may be the overarching voice of the poet. Here the poem introduces the eddy. The parenthetical is in language that clearly is separate from the more conversational diction of the passages in which the husband and wife speak to each other. The language in this parenthetical passage takes the reader into the rushing current of the brook and expounds on the white-water eddy. The passage reflects how the current of the west-running stream negotiates rocks along the bottom of the stream to form a frothy curling white eddy. It offers an elaborate metaphor: The curling white eddy that so clearly runs against the current is compared to feathers from the breast of some great white bird that fleck the surface of the black stream before being driven against the shore where it resembles something like a white scarf along the bank.