John Saunders (Mark Dightam) is the proud owner of two pet mice, named Father Christmas and Alice, and one of them is about to give birth; however, his mother has threatened to drown the babies because she doesn't want the creatures breeding, which leaves him only one option: take them to school with him. It turns out there's a trip organised for today to the Tower of London, so dutifully John accompanies them, clutching the box with Alice in it; however, he manages to lose her in the building, and becomes obsessed with getting her back...
What has all this to do with turning yellow? That bit arrives later, in this, a Children's Film Foundation which marked a small but significant footnote in the careers of a pair of the greatest talents to produce British films as part of the nation's golden age of moviemaking. They were Michael Powell (here directing) and Emeric Pressburger (writing and producing), creators of such classics as The Life and Death of Colonel Blimp and A Matter of Life and Death, in this instance in somewhat reduced circumstances. The CFF often hired talent whose best years were behind them, often it seemed simply because they were glad of the work.
But Powell and Pressburger, not receiving any other offers at the time, were happy enough to create a film for children, and even on a small budget managed to apply their imaginations to a little marvel of invention, hiring their old friend Christopher Challis for the photography. What happens to John is that he is sent home for falling asleep in class thanks to him being up all night worrying about Alice, and takes the London Underground to the station nearest his house. Now, if you think the teacher was derelict in his responsibility to let the boy wander off on his own, you would be proven correct because a calamity befalls John as he sits in that Tube carriage.
Calamity might be stretching it a bit because John appears quite pleased by the turn of events, but in a blink of an eye the whole train and its contents are transformed into a bright, sunflower yellow. Our young hero runs off and his mother calls the doctor, who can find nothing wrong other than the colour, and John is allowed to stay up and watch a TV documentary about the sudden phenomenon which has affected a number of people. Now, the television is important because in a way which anticipated Wes Craven's horror Shocker, the kid wakes up later that night to find there's a man, Nick (Robert Eddison, short for Electro-nick) who has travelled via skiing the electricity in the screen and may be able to help with the mouse problem.
That TV element is important because Pressburger sneaked an educational aspect into the plot where the children would be told about the possibilities, facts and dangers of electricity. We are informed about such things by John's friend Munro (played by future screenwriter Lem Dobbs - Powell was a friend of his family) and by Nick himself, who underlines the issues inherent in sticking your fingers in plug sockets which he can do for replenishment but you cannot because it will kill you. There follows a raid on the Tower to rescue Alice which sees, after an interlude at a football pitch which features the line "Hey ref, there's a mouse in me pants!", John arrested by the Beefeaters who really do eat beef and nothing but, and taken to be executed for trespassing. There was a lot of charm to this with its deliberately dreamlike plotting and matter of fact performances, so if you regret Powell and Pressburger never got another chance to make films for grownups again, you could at least appreciate their efforts to make a strange, sparkling movie for the kids.