As Chuck and Thomas walk towards the Homestead (Thomas decides that's what he'll call the run-down shack), Chuck decides to play a practical joke on someone using the bathroom. Apparently, he's done it before, but this time the poor sap was Gally. When Chuck runs away to safety, Thomas is left red-handed for Gally to catch. This just leads to another hostile interaction between the two, where Gally reasserts that he thinks he knows who Thomas is, and he's aiming to find out. Then he's gonna tell his Mom on him… or something like that.
Although this leaves Thomas feeling oddly exhilarated, while he's lying in his sleeping bag in a grassy meadow near the gardens, he returns to feeling melancholy and desperately curious.
Thomas tells Chuck that he wants to become one of the Runners, even though he still doesn't really know what they do.
Then, for some strange reason, Thomas is struck by a feeling of comfort and familiarity. He admits to his friend, "Chuck, I… I think I've been here before" (5.53).