The Runaway is the ninth episode ever made in Season 2.
One day, Thomas wakes up feeling ill. Worken are unable to make him feel better, so the Fat Controller is forced to send Thomas to the works for a while. He then asks Duck to take over Thomas' duties on the branch line. Duck is delighted to be given such a responsibility.
Duck is already friends with Percy and quickly befriends Toby, Terence, and Bertie, Terence tells Duck to take extra special care with Annie and Clarabel. Duck is very gentle with them and both coaches are left very impressed with the Great Western engine and remark on what a pleasure it is to go out with him. When Thomas returns from the works, the two coaches tell him how well Duck had managed on the line. Thomas does not feel jealous, he is just happy to be back home.
Although he is back from the works, Thomas is not in complete working order. His handbrake has been left very stiff, meaning it can feel like his brakes are hard-on when they are not. Because of this, Thomas begins to over run the station platform embarrassing him greatly. Eventually, Thomas' crew learns to compensate for the handbrake and they stop in just the right place.
Thomas' fireman falls ill one day and a relief man takes his place. Thomas arrives at the station and the new fireman couples him to Annie and Clarabel then joins the driver and stationmaster on the platform while they wait for Henry to deliver his passengers. However, the relief man has forgotten about the stiff handbrake. Soon Henry starts to approach with the passengers, but as Thomas' brakes are not completely on, his wheels start to move and Thomas cannot stop or whistle a warning without his crew. Before anyone realises what has happened, Thomas has left everyone stranded on the platform.
Annie and Clarabel scream for Thomas to stop, but he has plenty of steam and keeps going. Soon an alarm goes out about the runaway train. An engine inspector boards Harold the helicopter and flies to a station where Thomas is due to pass by and makes a plan.
When Thomas reaches the station, he is very tired and is moving far slower than normal. The inspector makes it to the platform, and when Thomas passes, the inspector scrambles aboard the cab and puts Thomas' brakes on, finally bringing the tank engine to a stop. Both Thomas and the inspector are very relieved and thank Harold, who tells them it was no problem.
The inspector tells Thomas that they must make sure this never happens again. Thomas, exhausted, could not agree more.
- When the Fat Controller speaks to Duck, the buffer of the breakdown train next to him is broken.