 Luke enters Clyde's painting for a 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 is unaware of this as she and Luke have argued over the untidiness of his room.While the class is at the International Gallery with its director, Mr Harding, something causes the image of Mona Lisa to come to life and step out of the painting, and to replace herself with Mr Harding's assistant, Miss Trupp.