Percy Takes the Plunge is the eleventh episode ever made in Season 2.
Percy is telling Bill and Ben the story of when he braved bad weather to help Thomas. Then Henry arrives and sends the twins away. Henry does not believe and scoffs at Percy's story. So Percy races away singing a song, reminding Henry the incident when he stopped in the tunnel to avoid the rain spoiling his paint.
Percy finds Thomas near a "DANGER" board. Thomas tells Percy he went down a mine after passing a "DANGER" board. Percy says he does not see a mine. He does not know that the foundation of the quay had sunk and that the rails lead into the water. Percy thinks the board is stupid and makes a plan to pass it.
While pulling some trucks, Percy asks them to bump them once they arrive at the quarry. His plan was to stop the trucks after they push him past the board, but any wise engine knows that you cannot trust trucks. Once at the quay, the trucks bump Percy, causing his driver and fireman to fall off the footplate. Percy slides past the board and plunges into the water. The Fat Controller is furious and Percy has to stay where he is until high tide.
The next day, Henry takes Percy to the works as a part of his train. Henry teases Percy by asking him if he liked the water. Percy simply answers "no" and Henry comments that maybe he will like it better next time. Percy doubts that there will be a next time.