Lady Hatt's Birthday Party is the fourth episode ever made in Season 5.
It is Lady Hatt's birthday and the Fat Controller is leaving work early to attend the party at Kirk Ronan. When his car becomes stuck in a pothole, the Fat Controller hitches a lift in Caroline. But the small car breaks down and when George comes along, he is given the job of transporting the Fat Controller, who quickly becomes covered in oil from George's engine. George loses control and crashes into a pond, and the Fat Controller is catapulted into the waist-deep mud. Thomas is luckily nearby and the Fat Controller acts as Thomas' "relief" fireman. The Fat Controller arrives on time, and the amused guests are quick to make good humour of the Fat Controller's adventure.
- When Caroline overheats, she lurches backwards and overheads again in slow motion.