The Monster Under the Shed is the fourth episode of the first season of Thomas' Adventures Chronicles.
When the Fat Controller closes the railway early due to bad weather, the engines are stuck at the station. While the engines wait to go back to the sheds, Thomas suggests a story to be told. James tells the engines a story.
A long time ago, a small blue engine was always ready to help. One night, the stationmaster asked him to retrieve a coach that was left behind. The engine went on, but when the weather turned bad he settled into an old shed. Later, he woke up to a noise, and when he opened his eye he saw fingers reaching for him from under the shed. The engine was chased out of the shed by a monster. He was never seen again.
Later that night, Percy wakes to a clanking noise behind his shed, but it is only James shaking some scrap. Through the night, Thomas hears creaking noises near the sheds. As a result, he stays up all night.
The next morning, Thomas is at Brendam with Percy, who had not heard any noises, making Thomas feel foolish for staying up all night. Henry, Twilight and Fluttershy feels bad for Thomas, while James teases him about staying up all night. Late Rarity came by and ask if they have seen Opal her cat anywhere but they haven't.
That evening, the engines are telling stories again, but Thomas is too tired to listen. He puffs straight to Tidmouth Sheds without stopping.
While the other engines are sleeping, Thomas hears the same noises he had heard the night before. He sees two glowing eyes, and thinks they are those of the monster. However, he finds that it is just Rarity's cat Opal, who have sneaked into Tidmouth Sheds. Rarity said she was sorry about what happened as she took Opal home and have a word with her. Thomas talks to Gordon about Percy's imagination.