Watt On Earth centres around the titular character Watt; an extra terrestrial with, by human standards, an odd taste in food, and the ability to transanimateobjectify. He comes to Earth to flee his evil uncle: Watt is heir to the throne on his home planet, and his uncle had sent his top henchman Jemadah to follow and kill Watt. While Watt is over 300 years of age, he appears to be a human in his early twenties.
Watt lands in the English town of Haxton, where he becomes friends with Sean Ruddock (played by Tom Brodie). Sean keeps Watt's presence in the house a secret throughout the series.
When they first meet, Watt is disguised as an apple with two stalks. Every episode features Watt transanimateobjectifiying into a different inanimate object, each time with an error. Watt himself has back-to-front ears in the first series, and green ears in the second. The first series ends with Watt losing the ability to transanimateobjectify, and in trying to regain the ability, he passes through each of the transformations in the previous eleven episodes.
Sean's parents are in charge of the local newspaper, The Haxton Weekly, and think their son is odd. Sean's sister, ZoŽ, is suspicious of his behaviour, and comes close to seeing Watt on many occasions. In the final episode of series two, she finally works out who Watt is, and turns him in to Jemadah.