- “Well, thank you, my dear. I knew this was my special birthday party, but I didn't know it was fancy dress!”
- ―Lady Hatt greeting her husband
Lady Hatt's Birthday Party is the fourth episode of the fifth series. It aired back to back with Sir Topham Hatt's Holiday on Storytime with Thomas in the US.
One Summer's Day, Bertie asks Thomas and Percy if they have noticed something different about the Fat Controller. Percy tells Bertie that he saw the Fat Controller staring into the clouds this morning and he wonders why. It is Lady Hatt's birthday and the Fat Controller has a brand new outfit to wear especially for the party. Lady Hatt tells him not to be late and he tells her that he will be spic and span and right on time.
The Fat Controller is leaving work early to attend the party at Kirk Ronan. He tells the stationmaster that he will be taking his car as the engines are busy. He thinks it is reliable, but it is not. His mood soon changes when his car becomes stuck in a pothole with a flat tyre. He decides not to risk changing it because it will cause his suit to get dirty for the sake of his wife's party.
The Fat Controller then sees Caroline approaching. He explains the situation and hitches a lift in her, but Caroline does not like going fast and her engine overheats, causing her to break down. George the Steamroller then arrives and insults Caroline, but his driver is more polite to the Fat Controller and offers him a ride to Thomas, who is just down the line and he agrees, promising Caroline that he will send for help. As George enjoys rolling along the road, the Fat Controller becomes covered in oil, thus ruining his suit. To make matters worse, George suddenly slips out of control due to a malfunction and ends up roller first into a muddy ditch not too far where Thomas is taking on water and the Fat Controller catapults and ends up waist-deep in the mud.
The Fat Controller asks Thomas' driver for a ride, but Thomas' fireman had taken ill, so the Fat Controller offers to be Thomas' relief fireman. He works hard and at last, Thomas arrives at Kirk Ronan just in time. The Fat Controller quickly buys a bunch of flowers for the party and as the clock strikes three, he arrives and gives his wife the flowers. She is most grateful and states that she knows it is her special birthday party, but not that it was "fancy dress."
- Sir Topham Hatt
- Lady Hatt
- Jem Cole (Not Named)
- James (Cameo)
- Mrs. Kyndley (Cameo)
- Refreshment Lady (Cameo)
- The Butler (Cameo)
- The Tailor (Cameo)
- The Railway Board (Cameo)
- The Ffarquhar Stationmaster (Cameo)
- Bridget Hatt (Deleted Scene Cameo)
- During the scenes where the Fat Controller rides in his car and Caroline, music is heard in the US and international versions but not the UK version.
- In the UK version, George's whistle is heard earlier than in the US and international versions.
- In the US and international versions, the sounds made by the Fat Controller's car, Caroline and George are barely audible.
- In the UK narration, Lady Hatt gains a classy accent.
- When the Fat Controller arrives at Lady Hatt's party, an instrumental version of the song "Happy Birthday to You" is heard playing in the background.
- Near the clock of the ballroom is the same portrait of a bearded man holding a top hat from Topham Hall in Thomas in Trouble, the Ffarquhar Stationmaster's house in Thomas Comes to Breakfast and Sir Topham Hatt's Office in Series 4.
- Also when the Fat Controller arrives at his wife’s birthday party, as the doors open, pictures of famed railway engineers Robert Stephenson (left) and Isambard Kingdom Brunel (right) can be seen in the background.
- This episode marks the first of three things:
- The first time Lady Hatt is referred to by name.
- The first time a human has been the main character in the television series.
- The first time the Fat Controller has had to be a engine's fireman, until Thomas and the Royal Engine, which was a callback to this episode.
- Funnily enough, both times the Fat Controller has had to do this, it has been as a relief fireman and also with Thomas.
- One of the flowers the Fat Controller buys is really a stem of broccoli.
- This episode marks the last of two things:
- Caroline's last speaking role to date.
- Jem Cole's last speaking role until the special, Journey Beyond Sodor.
- The same model previously used for Elizabeth II can be seen amongst the crowd at the party.