The Big Freeze is the Christmas episode of season 21 of Thomas and Friends.
It is after Christmas on Sodor, and everyone is enjoying the snowfall, but it causes delays on some deliveries, such as Paxton returning to the Vicarstown Dieselworks with fresh diesel fuel for Den and Dart. Meanwhile, Diesel has some fun bumping Thomas out from under the coal hopper before he can fill up, though Percy warns Thomas that the hopper is nearly empty. Thomas knows a fresh shipment is due any day. That night, the Fat Controller and Bertie nearly collide with each other on the icy roads after giving each other a friendly honk. As a result, the next morning, the Fat Controller visits the sheds to advise the Steam Team to be extra careful on the rails, administring speed restrictions and reduced services, especially having Gordon be assigned a back engine for Gordon's Hill.
His concerns are proven correct when Emily, after having her snowplough removed, and her coaches nearly collide with Toby and Henrietta due to frozen signals and points, leading to the signalman fainting in relief after seeing Emily and Toby managed to avoid colliding with each other. Henry derails on a siding due to frozen points, and James overshoots the platform at Knapford Station due to icy rails. By that evening, the Fat Controller is impressed by the Steam Team managing to keep things running and being Really Useful, but Percy is still worried about the coal delivery. Without it, the steam engines will be out of action until it arrives. The Fat Controller hopes it will be here by morning from the Mainland and tells the engines to keep their hopes up.
However, the next morning, Emily finds the hopper empty, as the coal was still late. As a result, the Fat Controller had ordered the Steam Team back to Tidmouth Sheds until further notice. Thomas, despite Annie and Clarabel telling him to listen to Percy, wants to finish taking his passengers to their stations. Unfortunately, he runs out of coal and has to stop to let his passengers disembark at Kellstorpe Road Station, stranded. As he and his coaches worry about how the Fat Controller will keep the railway running with the Steam Team out of action due to the coal shortage, Thomas' spirits are lifted when he hears the horns of the diesels and sees them coming up the tracks, but they past right through the station without stopping to help Thomas, his coaches, or his passengers, leaving them disheartened.
Den, Dart and Sidney soon return with the Express coaches to pick up Thomas' passengers, while the other diesels worked to keep the railway running. The rest of the Steam Team make it back to Tidmouth Sheds safely on their remaining coal with Gordon being the last of them. Percy is worried about Thomas, who remains stuck at Kellstorpe Road Station with Annie and Clarabel, freezing in the winter night air. Diesel passes by but refuses to help Thomas and his coaches, still smug at having more important things to do and being Really Useful.
The next day, it is still freezing out. Annie and Clarabel try to keep their spirits up by singing Christmas carols, and soon they and Thomas hear Diesel again, but believe that he is just going to leave them to suffer more until he reveals he came with the coal trucks to get the Steam Team back in action. Annie and Clarabel call Diesel their hero, and he is touched by this, while Thomas feels insulted after hearing that.
Thomas returns to the coal hopper, where the rest of the Steam Team are lined up to be reloaded with fresh coal, and Percy is relieved to see him after being so worried about him. With fresh coal, the Steam Team return to work, and the next day, things seem to be getting better as the big freeze has ended and things are starting to warm up around the Island. As Thomas passes by Diesel, he whistles to him, but as Diesel honks back with a huge discharge of fumes, upset at no longer being important with the Steam Team back to work, when he hears Annie and Clarabel still praising him for his heroism with saving them, he blushes and smiles before continuing on his way.