- “I'll have some of your floorboards please."
"I only swept the floor this morning!”
- ―Percy's Fireman and the Guard
Every Summer, the Island of Sodor is busy with holidaymakers visiting the Island to see the sights. With no sign of inclement weather, visitors often enjoy going to the mountains or the valleys, while children enjoy the seaside. One day, Thomas is busy on a coastal run taking children to the beach. When he reaches the next station where Percy is taking some trucks to the harbour, Percy tells Thomas he wishes that he could take children rather than trucks. Thomas states that the children he is taking are the Vicar's Sunday School, and he will be busy in the evening but then offers Percy to take the children back home, in which Percy agrees to do so.
At Dryaw, Harold warns Percy there is inclement weather and to run very carefully. At the next station, Edward warns Percy to be careful of an upcoming storm, but Percy knows what is going on. The children enjoy a fun day at the beach and arrive at the station, the weather turns bad and begins raining hard. Percy arrives at the station he came to earlier where Annie and Clarabel are glad when he arrives just in time to take the children home. Percy is disgusted by the rain, but despite thinking of being at the sheds, he manages to get on with it.
As Percy begins his return journey, he battles onwards through coastal villages and the countryside. The heavy rain makes the river rise very quickly. The visibility is poor for Percy while he faces the elements, but there is trouble ahead; the tracks have been flooded and as Percy plunges into the water, his fire is dampened as water floods into his cab. His crew then tries to find more firewood and his fireman asks the guard for some floorboards from the brake van. The guard grumbles that he had only swept the floor that morning but still agrees to help. Percy's fire is soon burning well and he feels more comfortable again. Then Percy sees Harold, who he thinks has come to laugh at him. A crate attached to a parachute is dropped out and thuds on top of Percy's boiler. His driver laughs and tells Percy that Harold has thoughtfully given the crew hot drinks. Percy thanks Harold before the helicopter flies away.
Percy bravely continues on with his journey and begins losing steam again, but keeps going. He makes several efforts and triumphantly brings the children home. Thomas congratulates Percy for keeping his promise despite everything. The Fat Controller arrives aboard Harold and thanks Percy and his crew. The Fat Controller is greatly impressed with Percy's efforts and praises him for being a Really Useful Engine.
- Sir Topham Hatt
- Annie and Clarabel (do not speak)
- The Vicar of Wellsworth (does not speak)
- James (cameo)
- Stephen Hatt (cameo)
- Bridget Hatt (cameo)
- Mrs. Kyndley (cameo)
- Jeremiah Jobling (cameo)
- The Little Boys (cameo)
- Harold's Pilots (cameo)
- Tidmouth Hault
- Tidmouth Beach
- The Watermill
- The Country Line
- Hawin Croka Canal
- The Flooded Track
- Arlesdale Lake and Mountains Line
- Dilly's Pond (deleted scene)
- Knapford Harbour (mentioned)
- This episode was originally planned to be a part of the second series, but was pushed to the third series.
- The episode can be seen in a Japanese game, Magical Adventure.
- In the restored version:
- A different angle of Arlesburgh is shown.
- An extended shot of Percy meeting Edward at Elsbridge is shown, with the last few seconds cut off.
- The fade effect and scene of Percy arriving at Elsbridge with Annie and Clarabel are removed and replaced with a cut and an extended shot of Thomas waiting for Percy.
- In a rare picture, Edward is wearing his exhausted/alternate worried or sad face mask instead of his neutral expression face mask when he talks to Percy at Elsbridge.
- There are noticeable differences in the set between the original and the restored episode when the narrator says, "Children love the seaside."
- Deleted scenes from the first series of Thomas on Toby's Old Tramway are used at the beginning of the episode.
- This episode was reran back to back with the third series episode, Henry's Forest on Storytime with Thomas.
- Modified models of the Coast Guard and a shrimper from TUGS appear at Arlesburgh Harbour, making this the first episode to use TUGS models.
- In the US and early UK narration when Harold and Edward warn Percy about the storm and when Percy returns to Tidmouth Hault to take the children home, the music is omitted.
- This episode was never released on Finnish DVD or shown on television. It was only released on a website in 2015.
- The Vicar of Wellsworth takes the role of the Vicar of Knapford.
- This is the first episode to feature Annie's raised front bufferbeam.
- When Percy plunges into the flooded tracks, Annie and Clarabel have swapped positions and the water appears to come above his firebox. However, in the next shot, the water is below his buffers.
- When Thomas is on Toby's Old Tramway, Clarabel is facing the wrong way. Then in the next shot, Annie is facing the wrong way.
- Despite the fact that the water only reached Percy's footplate, he has a water line across his boiler when he meets Thomas at the end of the episode.
- In the shot of Percy's driver and fireman during the scene where Percy is in the flooded water, the fence behind them is submerged; but in the close-up of the fireman, the fence is completely above the water. This changes again in the shot of the driver, fireman and guard carrying the wooden planks through the water.
- When Thomas talks to Percy at Tidmouth Hault, a red mark can be seen next to his left eye.
- When Percy says "I wish I could see," his roof is lifted.
- In the Polish dub, Edward's English name is used by accident.
- Harold: Sorry, Percy. Can't talk. I'm on high alert.
- Percy: Why?
- Harold: Bad weather's due. My help's always needed. Mind how you go, Percy.
- Percy: Ha! As long as I've got rails to run on, I can go anywhere, in any weather, anyhow. [leaves] Goodbye!
- Edward: Be careful. There's a storm coming.
- Percy: A promise is a promise, no matter what the weather.
- The Fat Controller: Harold told me you were a wizard. He says he can beat you at some things, but not at being a submarine. I don't know what you two get up to sometimes, but I do know that you're a really useful engine.
- Buzz Books - Percy's Promise
- Magazine stories - Percy's Promise, Going for a Swim! and Percy Gets Wet!
- My First Thomas Books - Percy and Harold
- My Thomas Story Library - Percy (discontinued)
- Books - Percy's Promise (book)
- Board Books - Percy gets Stuck
- Engine Adventures - Percy