Thomas is feeling unwell, so Edward takes him to the Works to be repaired. Whilst Thomas is away, Duck fills in for him and is very popular with Annie, Clarabel and the passengers. When Thomas comes back, he resumes his duties. However his handbrake hasn't been fixed properly by the workmen and one day the Fireman forgets to pull it hard enough. As a result, Thomas and the coaches "run away", and only Harold and an Inspector can help stop the runaway train.
