Giovanni is a young boy residing in a rural town evocative of early 20th-century Italy, burdened by familial duties due to his father's absence, implied as lost at sea or imprisoned. He serves as the primary provider for his bedridden, unnamedly ill mother and an unseen sister. This compels him to work after school, delivering newspapers and assisting a typesetter at a printing house to afford necessities like bread and milk. His poverty and adult responsibilities isolate him socially, making him an outcast mocked by classmates, especially concerning his father's absence and unfulfilled promises like an otter-skin coat. Campanella remains his sole friend and source of compassion.

During the local Festival of Stars, Giovanni attempts to fetch milk for his mother but finds none initially. Fleeing further ridicule from peers near the festival, he retreats to a hilltop where exhaustion and emotional distress trigger a visionary experience. He boards a mystical steam train traversing the Milky Way, encountering Campanella as his companion. Throughout this journey, they witness surreal astronomical phenomena: celestial fields, archaeological excavations on the Pliocene Coast, a bird-catcher transforming herons into candy, and victims of the Titanic disaster. Passengers share apples, a symbol of communal sacrifice, though Giovanni abstains from eating his. A child passenger tells the fable of the self-immolating scorpion, which profoundly impacts Giovanni; the scorpion’s choice to become a guiding light in death after a predatory life inspires him to vow eternal companionship with Campanella and dedicate himself to others' happiness.

As the train approaches the Southern Cross, most passengers disembark for a Christian-influenced heaven. Campanella glimpses a "true Heaven" (the Coalsack nebula) where his mother awaits and vanishes despite Giovanni's protests. Giovanni awakens on the hill, discovering Campanella drowned while rescuing a classmate, Zanelli, during the festival. Confronted by Campanella’s grieving father, who confirms the death and reveals Giovanni’s own father will soon return, Giovanni processes his grief by internalizing the scorpion’s lesson. He reaffirms his commitment to self-sacrifice and altruism, delivering milk to his mother with renewed purpose.

Physically, Giovanni is depicted as an anthropomorphic cat with dark blue fur, darker patches on his forehead and paws, and brown eyes, often wearing a red-and-orange vest. In expanded material, he develops "Star Tears" that heal wounds and sprout flowers, befriends characters like Sagwa through shared loss, and joins interstellar groups like Star Cat or the Honorary Toa Alliance, wielding advanced weaponry and piloting starfighters. These extensions emphasize his enduring association with cosmic journeys and empathy.