Thomas Keeps a Secret! is a magazine story.
One November morning, the Fat Controller speaks to Thomas in the main station yard. He asks if Thomas can keep a secret before sending him on a journey. James is curious and asks Thomas where he is going. Thomas says it is a secret before he puffs away. James really wants to know about the secret and so do the other engines when they hear about it.
Meanwhile, Thomas arrives at a factory yard. Men load big boxes into Thomas' trucks and then travel with him to a siding beside a large field near the main station. Terence is waiting; he knows about the secret, too. The men take the boxes from Thomas' trucks and put them in Terence's cart. Terence then pulls them into the field where they are opened up. The men then rope off part of the field.
Thomas returns to the main station where the Fat Controller is waiting for him. The Fat Controller thanks Thomas for helping with a special surprise for the railway staff and their families.
That afternoon, the Fat Controller tells his staff that Thomas is going to take them all to the firework display that evening. He adds that all of the engines will see the fireworks, too.
After work, the other engines stay up late in the yard while Thomas steams off with Annie and Clarabel. He proudly takes the Fat Controller, his staff and their families to the siding at the nearby field. The men from the factory are ready and they let off the fireworks they have brought. The display is amazing. Thomas gasps as he sees a sparking engine in the sky. The Fat Controller admits that he had kept that a secret even from Thomas.