John and his class go on a school trip to the Tower of London. While he is there he loses his pet mouse and vows to return and find her later. Back in school, he is not very attentive and falls asleep during a lesson about electricity so his teacher sends him home. On the 'tube' there is a sudden flash, and John, the train and all of the passengers turn yellow. With the help of Nick (short for 'Electronic') John learns about electricity, invades the Tower of London and saves his pet mouse ... or was it a dream.