John loses one of his pet mice, Alice, while on a school trip to the Tower of London.

Upset back in class, he is sent home by his teacher for not paying attention during a lesson on electricity.

Later that day on the London Underground, the train and everyone in it suddenly turns bright, vivid yellow.

The pair return to the Tower of London in an attempt to find Alice, but they are menaced by Yeoman Warders and John is threatened with execution.

Powell and Pressburger also brought in some of their old colleagues from the days of The Archers, such as cinematographer Christopher Challis and actor Esmond Knight.