In the vibrant land of Avalonia, bordered by towering mountains, Jaeger Clade and his teenage son Searcher Clade set out on a daring adventure to uncover new worlds. During their exploration, Searcher stumbles upon a remarkable green plant that emits energy. While excitedly deciding to return to Avalonia with this extraordinary find, Jaeger dismisses the idea and continues on his solo quest, fueled by frustration.
Fast forward 25 years, Searcher has made a significant impact by harnessing this miraculous plant, now known as Pando, as Avalonia's primary energy source. Together with his wife, Meridian Clade, they cultivate Pando, while their son, Ethan Clade, grapples with his own aspirations, feeling the weight of his father's legacy as he harbors a crush on his friend Diazo. One fateful night, the president of Avalonia, Callisto Mal, arrives aboard her airship, the Venture. She reveals that Pando is rapidly depleting, urging Searcher to investigate its origins. Accepting the challenge, he joins an expedition to explore a vast sinkhole where Pando's roots are believed to lie.
In a twist of fate, Meridian, frantic to catch up, hops into her crop-duster, while Ethan and their loyal three-legged dog, Legend, sneak onboard the Venture. As they dive into the sinkhole, the Venture is suddenly attacked by menacing red wyvern-like creatures, forcing a crash landing in a breathtaking underground realm. Separated from the group, Searcher and Legend encounter a fearsome creature known as a "Reaper," only to be saved by Jaeger, who reveals he has been living in this subterranean domain all along, seeking a path to cross the mountains.
Meanwhile, Ethan embarks on his own journey, evading capture by befriending a peculiar blue creature he affectionately names Splat. Eventually reunited with Searcher, Legend, and Jaeger, the group faces further Reaper attacks but is rescued by Meridian and Callisto, returning to the Venture. A clash of ideals unfolds as Searcher is determined to fulfill their mission, while Jaeger yearns to continue navigating the wonders of the Strange World. Struggling with these contrasting viewpoints, Ethan expresses his frustrations, yet after a heartfelt conversation, Searcher and Jaeger come to respect each other's ambitions.
Their quest leads them to a cluster of Pando's roots, where they discover the true peril awaiting them: the roots are under siege by the world's menacing creatures. When Searcher learns of Ethan's desire to explore beyond, he wrongly attributes this wish to Jaeger's influence, leading to a dramatic confrontation where Ethan leaps onto one of the Reapers. In a pulse-racing moment, Searcher flies after him, and during their reconciliation, they uncover that Avalonia rests upon the back of a colossal turtle-like creature, revealing that they've been traversing its insides. It becomes apparent that Pando acts as a harmful infection threatening the creature's heart.
Determined to save the creature, they race back to inform the expedition team of the urgent need to eliminate Pando. However, against their will, Callisto confines them to prevent interference with their mission, while Jaeger escapes to see the truth for himself. With the help of Legend and Splat, the Clade family breaks free. Searcher and Ethan venture toward the creature’s heart to clear a course for the Reapers, while Meridian reclaims command of the ship and persuades Callisto to join their cause. In a thrilling climax, Jaeger and the family unite to dismantle Pando, allowing the Reapers to obliterate it, thus rejuvenating the creature's heart and restoring balance to Avalonia.
A year later, the landscape has beautifully transformed; Ethan now shares a relationship with Diazo, as they explore the Strange World, capturing resources with friends. Avalonia has transitioned from reliance on Pando to using wind turbines for energy. Jaeger reconnects with his ex-wife, who has found love again in his absence, while the bond between Searcher and Jaeger flourishes. The scene gradually zooms away from Avalonia, revealing the majestic turtle-like creature gracefully gliding across an ocean-covered planet.
