The speaker is a mouse, talking in a warm, rustic voice about a small, contented life in a manor house. Crumbs, tea, a common nest and dreamless sleep are all they ask for, and poverty feels perfectly fine "for mice." Then comes a sudden, thrilling change: the mice find themselves taller, upright, white and proud, racing along a road in the dark on the stroke of the hour. The poem's rhythm speeds up here, with breathless repetition ("we beat, we beat"), to capture the joy of flight and power. The mention of a cinder-girl dressed in white suggests the poem is hinting at the Cinderella story, with the mice transformed for one night.
Afterward they are "mere mice again," and the mood turns bittersweet. They enjoy telling their tale, but the closing stanza is a complaint. Having tasted glory, they can't go back to being simply content. The speaker's question, "Why?", carries a quiet ache: freedom shown briefly can make an ordinary life feel too small.