Who's budgerigar dies of a broken heart?
Johnny Brent, home from school during a scarlet fever outbreak, manages to con a younger boy out of a magnet by trading it for an "invisible watch". The other boy's nanny accuses Johnny of stealing, which makes Johnny feel guilty. He runs away from home. After an older boy uses the magnet to cheat at pinball and Johnny is implicated, Johnny tries to get rid of the magnet. He meets an eccentric iron lung maker who is raising funds for the local hospital, and gives him the magnet to be auctioned for charity. The iron lung maker tells the story of the magnet at various fund-raising events, exaggerating wildly and portraying Johnny as everything from a spoiled brat to a Dickensian ragamuffin.
After he returns to school, Johnny sees the little boy's nanny and overhears her telling her friend about her budgerigar, which she says has died of a broken heart. Johnny mistakenly thinks she is talking about the little boy himself, and becomes convinced that he is guilty of murder. He hides in the back of a van which takes him to Liverpool, where he comes into conflict with the local boys. He wins them over by convincing them he is a fugitive from the police. He saves the life of one boy who had fallen through a disused pier. The injured boy ends up in an iron lung made by the man to whom Johnny gave the magnet. When Johnny visits the boy, he sees the magnet mounted on the iron lung and is reunited with the inventor, who is delighted to have found Johnny again. Johnny is awarded the Civic Gold Medal, which he gives to the magnet's original owner, clearing his conscience.
the little boy's nanny