On a school trip to the Tower of London, Hampstead schoolboy John Saunders loses his pregnant pet mouse Alice. He is still worrying about her during the next day's science lesson on electricity. Classmate Munro has all the answers but John, who's been up all night, falls asleep. Unimpressed, the teacher sends him home.

Taking the Underground home, John is playing with a pen torch when he, a fellow passenger and the entire train are turned bright yellow in a flash. Running home, he finds his horrified mother thinks it's a trick with paint. Scrubbing him, however, she discovers he bleeds yellow blood.

The doctor reckons John is perfectly healthy despite being off-colour. Newspapers and TV bulletins tell of a 'yellow plague' affecting 406 people so far, spreading for miles from an epicentre over Hampstead. One pundit thinks it might be a target for an extra-terrestrial visit.

That night John is wakened by a metallic voice from downstairs, calling his name. It complains it's hungry. Claiming to be inside the television, the voice urges John to switch it on. When John does so a strange yellow man, wearing skis, appears on the sofa. Introducing himself as "Nick, as in electro-nic", he declines a chicken drumstick and instead plugs his hand into the electrical socket to feast on "delicious" electricity.

Nick shows John how to travel on the waves of electricity, and together they jump into the TV set to visit a power station. John wants to look for Alice in the Tower, so they travel via the Beefeaters' television set. The Tower guards are watching a football match. When John misjudges the distance, Nick and John find themselves on the pitch. On their second attempt they reach the Tower. Nick - who cannot be seen or heard by non-yellow people - disperses the Beefeaters by setting off an alarm, letting John hunt for Alice using her husband, Father Christmas. A mouse sighting panics the terrified beefeaters and they capture John after a sword fight.

John is placed in a cell, awaiting beheading. Nick contacts Munro, who has an idea to free John. When John walks to the Scaffold at dawn he asks to watch television for his last request. When it is switched on, he and Nick escape through it. Nick drops John off at the science class where he fell asleep and promises to return the mice home. Awoken by teacher, John finds himself back to his normal colour, and has all the answers on the subject of electricity.

John returns home to find Alice, Father Christmas and nine baby mice safe in their wooden box, which is now bright yellow.