The narrator, J., actually tells two stories about Wallingford during the course of the book. The first one comes at the end of Chapter IX, when he remembers a previous boating outing with a cousin. He was following an old map, as it turned out, because he was waiting to approach the lock at Wallingford. He thought he had missed it somehow, though this scenario seemed impossible. It turned out that the lock had been done away with, and that he was much farther along the river than he had first thought. On the current trip, the friends reach Wallingford in Chapter XVIII, and no mention is made of a lock. J. describes it as “a rude, mud-built town” that “old-world masons knew how to build.”
The story you may be referring to comes in Chapter XVII, when George and J. go to an inn at Wallingford for some evening refreshment. They spy a huge mounted trout on the wall, and they wonder about it. J. describes first seeing it.
Then a pause ensued in the conversation, during which our eyes wandered round the room. They finally rested upon a dusty old glass-case, fixed very high up above the chimney-piece, and containing a trout. It rather fascinated me, that trout; it was such a monstrous fish. In fact, at first glance, I thought it was a cod.
No less than five men, including the innkeeper, enter the scene one by one. Each tells the two friends how he had once landed the trout. Afterward, J. says:
He was called out of the room at this point, and George and I again turned our gaze upon the fish.
It really was a most astonishing trout. The more we looked at it, the more we marvelled at it.
It excited George so much that he climbed up on the back of a chair to get a better view of it.
And then the chair slipped, and George clutched wildly at the trout-case to save himself, and down it came with a crash, George and the chair on top of it.
J. and George soon learned the truth of the trout. This is probably the most memorable story in the book.