Two outsider children who live across the street from one another become friends and set contests for each other, the prize for which feats is the trading back and forth of a lovely white stone.
Fia is a little girl who always gets teased; Hampus is a little boy who always gets into trouble. One summer, the two find themselves to be kindred spirits.
For their relationship, they assume new identities: Fia becomes Fideli and Hampus becomes Prince Perilous, a circus acrobat. Prince Perilous draws eyes, nose and mouth on the church tower clock.
The white stone falls, they are found, and the mushrooms are dismissed as harmless puffballs. On the first day of school, they are uneasy; neither wants his true identity to be revealed to the other.
But the stone is exchanged once more, and the charm succeeds: Fia and Hampus are themselves, but, having been Fideli and Prince Perilous, they are no longer afraid.
“The shadow of the tree lay like a dark hand on her head. Her white dress, which was a made-over petticoat, was like a princess-s frock - the kind of fine muslin frock worn by the slender queens of the lan of Egypt.”
Then Fideli goes without talking for a whole day, despite threats and pleas; Prince Perilous ties the circus elephant to the schoolteacher’s flagpole; then Fideli plays “The Nightingale” on the piano in the cafe.
As the adult world closes in on them, the children retreat to their secret sanctuary inside a dense lime tree, and, determined to die together, eat poison mushrooms.