Don't Tell Thomas is the 198th episode of Thomas and Friends.
It is winter on the Island of Sodor and Christmas is coming, which Thomas enjoys annually. Stationmasters are decorating the stations and the children are building snowmen. However, there has been heavy snow so Thomas has had to work hard to clear all the lines. Thanks to him, all of the engines' journeys and jobs are done on time. They are all so pleased, that they want to thank Thomas for his work with a surprise party.
After clearing the last lines of track, Thomas looks forward to seeing his friends, whom he misses. Thomas puffs into the station and sees Harold and Toby. When Thomas tries to say hello, Harold warns Toby not to let Thomas know about the planned surprise party. Thomas feels left out for not being part of the secret. When he arrives at another station, he sees Percy with colourful presents loaded in his trucks. Thomas asks him if they are part of the surprise and Percy bursts to tell Thomas about it but puffs away immediately, making Thomas cross.
Later that afternoon, Thomas waits at a signal and sees Emily with a Christmas tree loaded on her flatbed truck. Thomas asks her if it is part of the surprise, but Emily crossly tells him that it is something for her to know and for him to find out. She then puffs away and Thomas, while pulling Annie and Clarabel, follows her on the same line. Emily turns towards one of the branch lines. But, as Thomas approaches, the points change, making him stay on the main line.
Back at Tidmouth Sheds, everyone is excited about the surprise. Just as Thomas arrives, James and Henry warn the other engines not to tell him about it. Therefore, Thomas is still furious and upset that he does not know what it is, so he just exits to sulk in a siding. Later, it is time to show Thomas the surprise but the engines are unsure where Thomas has gone. Edward convinces Harold to find Thomas, which he does and tells Thomas about the surprise is for him as a thank you for Thomas helping them out. He then tells Thomas to go to Wellsworth, where he collects the children. Thomas soon arrives at a country village station and sees the whole station decorated out to be a Christmas party, which is being held especially for him.