The speaker rides a trolley through a gray, worn-down city and seems to know something is about to happen, something "they" are planning. He admits he doesn't much care. His life feels empty, the daily crowd is unpleasant, and he even toys with the thought that fewer people might not be so bad. The poem doesn't say what "they" are up to, so we are left to fill in the threat ourselves.
Then a small child's voice cuts through his indifference. It sounds like his own daughter's, and though he can't make out the words, he understands the fear in them. The tone shifts from bitter detachment to urgent instinct. He says he "didn't think–just did," and we never learn exactly what he did. That gap makes the ending tense and unsettling. The mood moves from numb and cynical to quietly heroic, suggesting that care for a child can break through even a deep weariness with the world.