Giovanni is a poor and lonely boy who lives with his bedridden mother and works after school at a printing press to help support their household. His father left on a fishing voyage long ago and has not returned, a fact his classmates use to mock him, especially the bully Zanelli. Giovanni's only friend is Campanella, a kind and popular boy who quietly shields him from the worst of the cruelty. On the night of the Star Festival, Giovanni runs errands for his mother but is publicly humiliated by Zanelli and his group. Campanella is present but can only offer a sympathetic glance. Heartbroken, Giovanni climbs a lonely hill on the edge of town and gazes up at the Milky Way. Dizzy with exhaustion and sorrow, he suddenly finds himself aboard a mysterious train racing through the stars. Seated across from him is Campanella, who appears damp and subdued. The two boys travel together through the galaxy, passing through stations such as the Swan and the Southern Cross. Along the way, they encounter other passengers: a bird catcher whose trapped birds transform into sweets, a party of Titanic passengers who perished in a shipwreck, and a young girl who tells the story of a scorpion that wished to sacrifice itself for the happiness of others. Giovanni learns that his ticket is a special pass that allows him to travel anywhere on the line, while Campanella's ticket is an ordinary one. As the train approaches the Coal Sack, the dark rift at the center of the Milky Way, Campanella suddenly sees a figure he recognizes as his mother in a distant meadow and vanishes from the carriage. Giovanni awakens alone on the hilltop, shaken and tearful. Returning to the town, he learns that Campanella drowned in the river while rescuing Zanelli, who had fallen in. Campanella's father confirms the death but also delivers news that Giovanni's father has sent word he is coming home. Overwhelmed by grief and a new resolve, Giovanni races home to tell his mother, carrying the milk he finally obtained, determined to live for the true happiness of others, just as the scorpion and Campanella gave themselves for someone else.