Just across the Mersey from the Blitz-scarred city of Liverpool is the affluent town of Wallasey, where - when not at boarding school - young John Brent lives with his housewife mother and his father, a successful psychologist in the metropolis.

Currently at home due to an outbreak of scarlet fever at his school, John is a typical healthy young lad of about 11 years of age: he has a mind for adventure, an interest in invention, and enjoys spending his time in the great outdoors. Playing on the beach one day he encounters another, slightly younger boy, who possesses a large and very strong magnet. Offering a range of items from his own pockets in hope of a swap, the boy never the less refuses them all - until Johnny offers up his "invisible" pocket watch!

The swap is done and both boys are initially happy with the deal, but when the other youngster, Kit, returns to his Nannie, she is much less amused and confronts Johnny, accusing him of theft. Running away, he soon finds himself haunted by guilt at his deceit and sees policemen and other authority figures everywhere.

Hoping to discard of his ill-gotten gain, he peeks down a grate only to discover it is not in fact a drain, but opens to the cellar of an enthusiastic inventor of sorts who is constructing a model iron lung in order to demonstrate the purpose of such a contraption for a fundraising drive in aid of the local, non-NHS, hospital, St. Valentine's. Engaging in conversation with the gent, Johnny offers him the magnet as his own 'contribution' to the cause, which is kindly - if not amusedly - accepted.

However, Johnny's guilt is far from assuaged and he soon comes to believe that young Kit has died of a broken heart, bereft at the loss of such a powerful and wonderful thing as the magnet! With his imagination running apace he believes the police to be hunting him down on a possible murder charge: in fact, the entire community is soon on the look out for young Johnny, but only because the story of the magnet as a gift toward the hospital has been rather embellished and ended up a significant boost to the fundraising!