The poem imagines life as a play whose scenes are already written. Everything is "set to be played," and even the setting and stage have been put in place by someone else: an old man with "dark widespread arms." He is never named, so he could stand for fate, death, or some larger force that controls the show. The reader has to decide.
The speaker seems to feel powerless. The verbs are passive, and the final line shows the speaker being swung "deeper and deeper within the earth," like a child pushed on a swing, except this motion leads downward. That image is oddly gentle and unsettling at once. The broken punctuation in the last line, with its stray full stop after "who," makes the sentence feel like it stumbles before the pull begins.
The mood is quiet, fatalistic, and a little eerie. There is resignation more than panic, as if the speaker has accepted a script that ends underground.