The Great Discovery is a feature-length direct-to-DVD special.
The railway is preparing for Sodor Day and Thomas and James are racing to the wharf to be given a special job by Mr. Percival. Thomas wins and is sent into the hills to collect lumber with Duncan. After Thomas teases Duncan, Duncan pays Thomas out by sending him on an old, disused line which he claims will lead Thomas back to the wharf faster. After avoiding an old, collapsing bridge, Thomas takes another route at the old junction, and as he journeys further down the line, he comes across an old overgrown town.
The news of Thomas' discovery spreads, and eventually reaches the Fat Controller, who tells Thomas that the town is Great Waterton - the largest town on Sodor when steam engines first came to the island. The Fat Controller decides that the restoration of the legendary lost town would be perfect for Sodor Day and calls the Pack to help. Thomas is told to take charge of the engines working around the town, while a new tank engine named Stanley is brought to do Thomas' regular jobs.
All of the engines take to Stanley, who does a good job and has fun as well, but when Thomas sees Stanley doing his jobs and fraternising with his friends, he becomes jealous. Thomas is furious when Stanley takes his place at Tidmouth Sheds, and the final straw comes when Stanley takes his place as the leader of the restoration project after Thomas has an accident.
Later, Thomas is shunting stone trucks for Stanley to take away when he decides to make Stanley look foolish by giving him a train too heavy for him to pull, but the plan backfires - as Stanley ascends the hill, a coupling breaks and the trucks race down the hill, veering into a siding, and crashing into the newly-finished tower. The Fat Controller states that this will likely set back the restoration past Sodor Day, and Thomas is blamed. Stanley, however, realises that Thomas merely missed his friends, and wants to set things right.
That night, Thomas decides to clear the rubble out of the way. He is almost finished when he accidentally bumps a truck into an old mine shaft. Racing in after it, Thomas crashes onto a wooden platform and floats down an underground waterway. The engines soon realise that Thomas is missing, and the Fat Controller organises a search effort. Meanwhile, Thomas continues floating until he reaches an opening to the mine, and is flung across a canyon and down a hill.
Stanley decides to make another search effort by himself before he returns to work. Thomas, with the last of his steam, sounds a loud, long whistle. Stanley hears it and hurries to where Thomas is stranded beside the track. The two make up for their quarrel and Stanley pulls Thomas back onto the rails, but the strain is too much for Stanley and a valve bursts. Thomas, using some of Stanley's coal, pushes him back to Great Waterton. The word soon spreads that Thomas has been found.
The standpipe is rebuilt, completing the restoration. As a sign of friendship, Thomas offers Stanley his last job - bringing the mayor to the festivities at Great Waterton. The engines gather at the town for the celebration and Thomas realizes just how important his friends are.
- Max and Monty
- Sir Topham Hatt
- Lady Hatt
- Miss Jenny
- Mr. Percival
- Bill and Ben (do not speak)
- Harvey (does not speak)
- Molly (does not speak)
- Mavis (does not speak)
- Salty (does not speak)
- Skarloey (does not speak)
- Rheneas (does not speak)
- Sir Handel (does not speak)
- Freddie (does not speak)
- Mighty Mac (does not speak)
- Annie and Clarabel (do not speak)
- Rocky (does not speak)
- Terence (does not speak)
- Bertie (does not speak)
- Trevor (does not speak)
- Oliver (does not speak)
- Ned (does not speak)
- Jeremy (does not speak)
- The Mayor of Sodor (does not speak)
- Rosie (cameo)
- Whiff (cameo)
- Peter Sam (cameo)
- Rusty (cameo)
- Diesel (cameo)
- 'Arry and Bert (cameo)
- Diesel 10 (cameo)
- Dennis (cameo)
- George (cameo)
- Butch (cameo)
- Buster (cameo)
- Stephen Hatt (cameo)
- Bridget Hatt (cameo)
- Jem Cole (cameo)
- The Refreshment Lady (cameo)
- Lord Callan (cameo)
- Sodor Brass Band (cameo)
- Kuffy the Clown (cameo)
- Henrietta (music video cameo)
- Elizabeth (music video cameo)
- Kelly (music video cameo)
- The Great Discovery was shown in select theaters in the US throughout July, in the UK throughout September, and in Australia throughout November.
- This was the final Thomas and Friends production to use an all model set. The twelfth season aired with several parts of the set in CGI, and starting with Hero of the Rails all footage was done in CGI.
- Stock footage from various eleventh season episodes are used.
- In Japanese releases, this runs under the name of "Mystery Mountain".
- Pierce Brosnan was to take over as narrator starting with this special, but after HiT Entertainmentannounced its plans to switch to CGI, his status was changed to "special guest narrator".
- Among other things, the Sodor Daily newspaper reports that "Percy is in tears", and mentions something about "Trevor the Traction Engine". Another article reports on an Alicia Botti concert. The paper's articles are apparently written by "J. K. Lee".
- This special was broadcast on PBS Kids Sprout on June 5th and 6th, 2010.
- This special was broadcast on ABC2 in Australia on April 24th, 2011.
- This was the first US DVD to be released by Lionsgate.
- Special Wal-Mart editions, including a bonus CD featuring all five songs from the special, were released.
- When Thomas and James race to the Wharf, the number of lines between them changes.
- When Thomas and James enter the wharf, Freddie and Skarloey appear twice in two different places.
- There was no need for Stanley to come to Sodor to do Thomas' work as there are other tank engines on Sodor like Duck, Oliver, Rosie, and Billy available.
- Stanley's siderods are out of alignment when his wheels spin as he struggles with the trucks.
- When the engines are shown covered in brick dust, Toby's face looks crooked.
- When Stanley returns to Great Waterton after the accident he's on the track to Thomas' left. In the next scene, when Thomas is alone in a siding, Stanley is on the track to Thomas' right.
- When Stanley leaves Great Waterton to look for Thomas, James is seen at the back of the group of engines. After Stanley realises that Thomas has been feeling upset James is seen behind the line of stone trucks.
- When Thomas sees the barrier in the disused mine he brakes, but before he smashes through the barrier his wheels move.
- When Thomas falls off the bank the planks of wood that also fall are bigger than him.
- It is never revealed what happened to Thomas' crew after Thomas began to float down the river.
- When Thomas tips over, you can clearly see a bent piece of rail used to push him up and off the rails. His undercarriage is also visible; it appears too basic to be realistic.
- Thomas' crew should have been seriously hurt when Thomas rolled over.
- When Thomas comes to a stop at the bottom of the hill, the grass goes over his face, but in every following scene, the amount covering his face is different.
- After Stanley pulls Thomas onto the tracks, look behind Thomas. You'll see points that lead to where Thomas landed; this proves a hidden track was used to help Thomas onto the line.
- After Thomas is pulled back onto the rails he becomes clean.
- Thomas whistles after Stanley cracks a valve, but the narrator said that Thomas had no steam.
- Arthur tells Emily that Thomas was found, but Emily already knew.
- At the opening of Great Waterton, Gordon and Henry swap places twice by the standpipe.
- Bertie has Caroline's horn sound at the end.
- Wouldn't Thomas be too heavy to float through the flooded mine?
- The tower is rebuilt in one day. But, in reality, it would have taken longer to rebuild.
- A brakevan should have been at the end of Stanley's train.
- Surely, Thomas should have pushed Stanley to the repair yard when Stanley had a burst valve.
- Thomas was constantly going up and down inside the mine which would've been too dangerous as the water in his boiler wouldn't cover the crown of the firebox.
- When Stanley pulls Thomas back on the tracks a thin wire is seen pulling Stanley.
- On back cover of the US DVD, all of the engines are sad, yet there are balloons floating around.
- On the UK DVD, in the game "Follow the Whistle", Gordon is in Edward's place.
- How could Thomas whistle to Stanley if his fire was out?
- Thomas and James are Racing
- There's a Job for Everyone
- Where, oh Where is Thomas?
- Engine Rollcall
- Thomas, You're the Leader
- Behind the Scenes with Pierce Brosnan
- Spot the Difference
- Tough Trucks!
- Thomas' Puzzle Parts
- Thomas: I am brave and I am strong. I'll get to the Wharf and it won't take long!
- Thomas: I'm going to win!
- James: No, I will!