After defeating Phaethon, who set the world ablaze with the chariot of the Sun, Jupiter descends to Earth with Mercury to restore life to nature. There they encounter Callisto, a nymph devoted to Diana, with whom Jupiter falls madly in love. Callisto rejects his advances, as her devotion to Diana demands chastity. When the nymph departs, Mercury suggests that Jupiter impersonate Diana to get closer to her. The deception works: Jupiter, disguised as a goddess, gains Callisto's trust, and she yields, believing that Diana loves her. Meanwhile, the real Diana dedicates herself to reforesting the forests after the great fire, accompanied by her faithful servant Lymphea and closely followed by the shepherd Endymion, who is in love with her. Diana reciprocates his feelings, but her chastity compels her to resist. After driving Endymion away, Diana and Lymphaeum encounter Callisto, who, still deceived, goes to the goddess, reminding her of her recent love. Indignant, Diana banishes the nymph, leaving her alone and despairing. Pan is also in love with Diana: tormented by her rejection and fearing a rival, he orders his satyrs to patrol the forest to discover the truth.
On Mount Lycaeus, Diana finds Endymion asleep and, upon waking, confesses his love to him. However, one of Pan's satyrs spies on them and runs to inform his master. Meanwhile, Juno, Jupiter's wife, descends to Earth, suspecting yet another infidelity. She finds Callisto, still distraught, and, upon hearing her story, understands Jupiter's deception: he has disguised himself as Diana to seduce her. The arrival of Jupiter and Mercury, the god still in disguise, confirms Juno's suspicions. Callisto, torn but in love, gives in again. Jupiter arranges to meet her at the springs of Ladon, and the nymph goes there. Juno reveals herself and confronts Jupiter and Mercury; Jupiter's attempt to maintain the deception even before his wife increases the goddess's anger, and she storms off. The situation is further complicated by the arrival of Endymion, who goes to the false Diana, and immediately afterward by Pan and his satyrs: the god of the woods captures the shepherd and threatens revenge. Jupiter and Mercury flee, abandoning Endymion to his fate.
Callisto arrives at the springs of Ladon to await her beloved, but instead of Diana, Juno arrives, accompanied by the Furies, who transform her into a bear. Before departing, Juno orders the Furies to protect her, but the arrival of Jupiter and Mercury drives the infernal gods away. Moved by compassion, Jupiter promises Callisto that, after her death, she will be welcomed among the stars, despite Juno's wrath. Meanwhile, Diana frees Endymion from Pan's clutches and drives the forest god and his retinue away. Humiliated, Pan decides to take revenge by publicly revealing Diana's secret passion. In the sky, Callisto is transformed into the constellation Ursa Major.
