Luke enters Clyde's painting for an art competition and Clyde wins first prize. The prize is a trip to see the first showing of Leonardo Da Vinci's Mona Lisa in Britain for Clyde and his class but Sarah Jane Smith is unaware of this as she and Luke have argued over the untidiness of his room. While the class is at the International Gallery, something causes the Mona Lisa to come to life and step out of the painting, and replaces herself with somebody else. She also has the ability to make other people and objects from paintings come to life and able to put make real people become part of paintings, and does this with some gallery staff. Believing the painting to be stolen, the gallery calls the police, who arrive but who have not been there long before the Mona Lisa puts them too into paintings. She removes a Sontaran blaster gun that Clyde has unwisely painted from his winning picture and commands the gallery's director, Mr Harding, to take her to find her 'brother'. Seeing the coverage of the apparent theft of the painting on television, Sarah Jane Smith arrives at the gallery and argues with the Mona Lisa to let the painting people come back to life. Instead, she puts Sarah Jane into a painting.
The Mona Lisa tries to unleash a beast in the gallery basement, which she refers to as her brother. This creature is locked up in a vault and can only be released by using a puzzle box key. This key is destroyed by Harding who, despite his admiration for La Giaconda, cannot allow her to unleash the beast on the world. Luke, Clyde, and Rani have to try to solve the problem without Sarah Jane as she is now trapped in a painting. The Dark Rider captures Clyde and takes him to the vaults along with the Mona Lisa and Harding. As Mr Harding has destroyed the original key, Clyde is encouraged to draw another, that the Mona Lisa can bring to life as a replacement. This is done and the key is used, but before the beast can escape, a picture of K-9 drawn by Clyde also comes to life and fires at the beast, causing it to retreat into the vault. This sets off a chain reaction causing all the painting people to revert to being just works of art, and all those trapped in paintings to be released. The story ends with the Mona Lisa restored to her frame and Luke promising to tidy his room.