The speaker is a cleaner in what seems to be an institution for children, perhaps a dormitory or boarding facility, though the poem never names it. The opening admits a kind of innocence: with no children of his own, he assumed all children laughed in that "teasing, cruel" way. Now he sees what that laughter can mean, a scapegoat made to swallow something unspeakable, and blood on the shower walls.
The mood is tense, numb and quietly angry. His stomach clenches "like a fist / around a blank coin," a sharp image of fear and helplessness. He knows the state inspector will arrive and hear only the squeak of polished shoes, so the truth will stay hidden. All the speaker can do is work. Moving backwards down the corridor, mop in hand like a helm, he steers through "an endless river of filth," which suggests the mess is physical and moral, and far too large for one person to clean.