Canterlot High School's Terrible Swimming Pool Secret is the first episode of the third season of Thomas' Adventures Chronicles.
After Thomas accidentally crashed into the wall of the Canterlot High School while delivery a goods train and his brakes weren't working, The Fat Controller and Principal Celestia discovered that Canterlot High has it's own Swimming Pool (which has been closed down for years). The two decided to reopen the Swimming Pool for good. However strange things began to happen since it's been reopened. Students began to smell something terrible, followed by a grab on the leg as if students were being pulled underwater by a person (one that wasn't there) but they managed to get out of the water safely.
Later one night Thomas, Twilight, Ash, Mickey, Dan, Sunset Shimmer and their friends came to have a look at the pool and wonder what was causing strange things to happened. Percy suggest it might be a ghost. but Dan thinks Percy is being silly. However Misty and Brock knew Percy has a point. So Ash volunteer to put his hand in the water. but nothing happened. Percy to have admit in defeat that he must have been wrong until a strange stinky smell appeared then all of a sudden Ash was being pulled into the water. As Ash was beginning to drown, a mysterious man appeared handing him the long net and shout at him to grab as quick as he could. The man helped Ash out of the water just in time.
While Ash was drying up, The others discovered that the mysterious person is called Charlie an old lifeguard of the Pool House. He tells them when the school's pool first opened he was the lifeguard. Also during his watch his girlfriend Cindy's little brother Joe drowned, But it was not an accident. He says he tried to save the boy but the creature wouldn't let him. He pulled him away and then drowned him and he was gone. Charlie is really upset and is about to cry now, when he says that nobody believed him until three more people drowned. Then finally the closed off the swimming pool, and soon it was all forgotten.. until now.
Sunset Shimmer and Mickey ask Charlie what was it Charlie says that to build the pool they had to remove an old cemetery first and moved all the bodies, and he figures they forgot one of the bodies. Therefore the spirit is angry and wants revenge on anyone who dare disturbs it's eternal sleep. Clorice thinks that the same thing is going to happen all over again and no one will believe them until someone is hurt. Thomas knew this was all this fault he haven't crashed into the Pool House in the first place. But no one was blaming him for it and told him it was just and accident. Dan suggest they should get rid of this spirit once and for all, until Misty and Runo ask how will they do it? Sunset Shimmer had an idea to defeat the creature. They head to the science room. she explains since it's invisible, so no one is ever going to believe them. But if they can see it maybe they can stop it. Since it's part acid her plans to douse it with Methyl Orange, because when methyl orange reacts to something acidic it turns it red.
Sunset Shimmer puts on gloves and safety googles and pours some Methyl Orange into the pool. The red ooze slowly begins to take shape. Then suddenly emerging from the water is a hideous and terrifying zombie, that is a partially decomposing, part skeleton and all colored red. Everyone was in horror and Charlie seeing it for the first time after all this years begins to go into shock. The corpse comes towards them and Ash tries to whack it witch the handle of the net, but it just splashes right through it like it's nothing. They runs off to the change room where Brock is treating Charlie who is lying down in shook. Twilight explains that since The Corpse is now visible, it can't hide anymore and now roams freely. Also since it's made of water it can travel through pipes and drains and instantly appears in the change room through the drain on the floor. They run off screaming.
The Others doesn't know how to stop it but Sunset Shimmer has another idea. Since The Corpse is water based, She says she'll douse it with mangenite since it's water based it'll explode. Rainbow Dash applauds her for remembering his experimental volcano and says she is brilliant. Sunset Shimmer is about to open the container, when Ash slaps it away and it sinks to the bottom of the pool. He did this because her hands were wet and if she opened it the chemical would burn her skin. Misty swim in after it, she swims to the bottom of the pool to retrieve it. Then The Corpse slimes back into the pool and begins to move towards her. Everyone screams her name to her and warns her to get out. She sees it moving in on her and she is frozen in fear and shock and can't move. Thomas plunges into the water to save her and when he does she snaps out of it. The Corpse is now after him and Percy quickly pulls him out. Misty makes it out if the pool with the chemical and Sunseet Shimmer puts on the safety gloves.
Percy pulls Thomas out of the water just in time and Sunet Shimmer throws the chemical at The Corpse loudly and violently screams and moans as it bubbles and oozes and sizzles and smokes and steams and evaporates away into nothing. Zeke applauds her for defeating it and they smile in their victory and Charlie looks on in relief because finally after all these years the nightmare is finally over and gratefuly thanks them.
- Thomas, Percy, Twilight Sparkle, Spike, Sunset Shimmer, The Human Mane 5, Ash Ketchum, Pikachu, Misty, Togepi, Brock, Mickey Mouse, Donald Duck, Goofy, The Bakugan Battle Brawlers and their Bakugan, The Fat Controller and Principal Celestia are starring in this episode.
- This episode is based on Are You Afraid of the Dark episode - The Tale of Dead Man's Float.