Sammy is small for his 12 years but is always working out - he hopes one day to fill his oversized 'lucky' T-shirt, which bears a tiger motif. While his 'friend, manager and trainer', Marvin, registers Sammy for the Junior Superstar Challenge Cup Race, bullies Big Sid and Chalky tease Sammy, throwing his favourite T-shirt through the open window of Chemi-Tex's laboratory, where it lands among old shirts used by research scientist Trotter for fabric treatment experiments.

When Trotter makes a breakthrough in flame retardancy and rushes off to tell his boss, Mr Becket, Sammy and Marvin sneak into the lab. Alarmed to see his tiger T-shirt in some sort of electrical ray treatment cabinet, Sammy stands on a chair to fish it out but tumbles off, tearing electrical connections and causing a flash of power to strike the shirt.

Returning, Trotter shoots the tiger T-shirt with a rifle and is amazed when the shots simply bounce off it. Its entire molecular structure has changed - the hyper-dynamic field not only absorbing energy but multiplying it, potentially giving the wearer super strength. Becket and Trotter leave the lab, plotting to sell the secret to the highest bidder. The boys emerge from hiding and Sammy takes back his T-shirt. A locked door prevents their exit but Sammy pulls hard and rips off the handle with new-found super strength.

Sammy easily lifts Marvin over a high wall, which he then vaults himself. Running off, Sammy meets a little boy whose ball has been stolen by Sid and Chalky. He sorts out the bullies with his new powers.

Sammy takes the shirt to the laundrette. It drives the washing machine crazy, foam spurting everywhere. Trotter uses the confusion to snatch the shirt, bundling Sammy into a van and locking him up in a deserted old house where he and Becket are experimenting in secret.

Marvin is caught snooping and is locked up alongside his friend, but the pair escape through a window. Sneaking past Trotter's lab, they steal the super T-shirt and flee, escaping in a rowboat powered by Sammy's bare hands at super speed. Marvin's quickly drawn decoy T-shirt delays their pursuers as Sammy heads for the big race.

Sammy roars into the lead, assisted by the T-shirt, but it is now dangerously unstable, freezing Sammy in his tracks. As runners pass him, Marvin urges Sammy to believe in his training and take off the shirt. Throwing it aside, Sammy races on. He breaks the tape just ahead of Big Sid to lift the cup.

Becket and Trotter seize the discarded T-shirt, which promptly explodes, leaving them blackened and bruised. The ragged tiger motif appears to grin like a Cheshire cat as a hollow, mocking laugh rings out.