In the vibrant city of Yolcos, 80 years have passed since the legendary voyage of Jason and the Argonauts, yet the Golden Fleece continues to cultivate peace and prosperity for both the townspeople and the local animal inhabitants. Among them is Pattie, a spirited young mouse who idolizes the famed hero Jason. She nurtures dreams of embarking on her own thrilling adventures, much to the concern of her adoptive cat father, Sam (Christophe Lemoine), whose primary wish is to keep her safe. As Pattie matures into a young adult, her dreams seem to dwindle further away. Her aspiration to visit the grand library in Alexandria is abruptly dashed when, on the very day she wins a lottery ticket for the trip, Sam accidentally eats it.
Amid these personal tribulations, Jason announces plans to erect a grand statue honoring Zeus (Pierre Richard), the deity who ensures the community's protection through the golden relic. However, Zeus's incessant boasting earns him the wrath of Poseidon (Paul Borne), who, angered by the hubris, demands the citizens construct a statue for him complete with a shimmering trident, threatening them with dire consequences in the event of noncompliance, including a catastrophic tsunami.
The abrupt disappearance of the Golden Fleece plunges the community into chaos and despair. During these tumultuous times, Pattie stumbles upon a map leading to Trinactos, an island rumored to be rich in treasures, including a magnificent sapphire resembling a trident. This leads her to encounter Chickos, a cunning pirate seagull who confirms the island's existence. Sam hands the map over to Jason, who rallies the townsfolk to unite and construct Poseidon's statue while embarking on a perilous quest to Trinactos.
Using a mystical dragon's tooth, Pattie manages to resurrect the long-lost Argonauts, and in recognition of her intelligence, Chickos invites her along to assist in guiding the Argonauts, including Jason, on this formidable journey. However, motivated by concern for Pattie's safety, Sam inadvertently joins the expedition as well. As they sail, the gods, especially Zeus, throw hurdles in their path, including ferocious storms and the unleashed terror of a baby Kraken.
> In a comedic turn of events, the Kraken’s sneeze creates havoc on their ship, leading to Sam’s queasiness that uncovers the Alexandria ticket, which devastates Pattie.
After sustaining significant damage, the crew makes an unscheduled stop in Syracuse, where a heartbroken Pattie decides to disown Sam. Regret-ridden, Sam seeks assistance from local gangster rats to reach Trinactos, while the crew continues their journey, arriving only to find themselves ambushed by a tribe of Cyclopes determined to capture the Argonauts for dinner. Pattie and Chickos pursue the Cyclopes and uncover a forge, while Sam arrives on the island, where he meets two young mice named John (Aloïs Le Labourier Tiêu) and Terry (Juliette Davis), who introduce him to a trio of enchanting scorpions—Bernardo (Frantz Confiac), Gerardo (Jérôme Pauwels), and Theresa (Magali Rosenzweig).
As the story unfolds, Pattie realizes that the map was a deceitful trap orchestrated by the Cyclopes. She witnesses evidence of prior adventurers who fell victim to the Cyclopes’ malicious ploy, fueling her determination to set things right. In a tense confrontation, both Pattie and Chickos are attacked by a fearsome hydra but are miraculously saved by Sam, who sacrifices himself and is subsequently captured by the Cyclopes’ leader, Grumos (Serge Biavan).
Back in Yolcos, citizens face almost insurmountable despair, but Luigi, Pattie’s loyal friend, rallies their animal community to unite and deliver water to the humans, ultimately building harmony between the two factions. Returning to Trinactos, Pattie reconciles with Sam, who apologizes for keeping her sheltered. He acknowledges that her destiny was never meant to be conventional, making her feel confined. In an emotional moment, Pattie forgives Sam and frees him from captivity.
With newfound resolve, Pattie devises a plan to rescue the Argonauts and forge their very own trident using the forge. This leads to an exciting showdown where Chickos, John, and Terry commandeer the Cyclopes' massive robot as Pattie, Sam, and the scorpions face off against the hydra. The group manages to circumvent the Cyclopes, causing them to turn into trees at sunrise in a climactic stand-off. Grumos makes a desperate last attempt to halt their escape but meets his end when Pattie tricks him into standing in sunlight, transforming him into a tree as well.
In the final act, Sam adopts John and Terry as his own, and together the crew returns to Yolcos, narrowly escaping Poseidon's wrath just as he was about to unleash a tsunami. Grateful for the construction of his statue, Poseidon relents, restores the Golden Fleece, and peace returns. While Jason is celebrated as the town hero, the animals honor Pattie as the true savior of Yolcos.
In a delightful mid-credits scene, Sam and the rats present Pattie with her ticket to Alexandria. As their adventure continues, they encounter the gangster rats again, who remind Sam of his debt from the favor he requested to reach Trinactos. Introducing Graziella, a rat with a notoriously awful singing voice, they embark on an unexpected journey together, as Sam, John, and Terry stow away with Pattie bound for Alexandria, with the gods already plotting further challenges ahead.
