In Avalonia, a land surrounded by an endless wall of mountains, Jaeger Clade and his son Searcher are adventurers who brave the wilderness to explore new worlds. While trying to traverse one of the mountains, Searcher discovers a plant that gives off green energy. Jaeger insists on continuing the quest, but Searcher and the rest of the expedition team believe that the new plant, dubbed Pando, is good enough. Jaeger angrily continues his mission alone. Twenty-five years later, Searcher has made a name for himself by harvesting Pando and turning it into a fuel resource for Avalonia. He is married to fellow harvester Meridian and they have a son named Ethan who gets embarrassed whenever Searcher tries talking to his crush Diazo.

One day, Callisto Mal, the leader of Avalonia who was one of Jeager's former expedition teammates, shows up in a giant airship called the Venture and informs the Clades that the Pando is losing its power, meaning that they need Searcher to help find the cause. He joins a new expedition team to travel into a giant sinkhole where giant roots of Pando have been located. On the way down, Meridian tries catching up in her crop duster after Ethan and their dog Legend have sneaked aboard the ship. Ethan wanted to come with Searcher but was shortlasted when they are attacked by red wyvern-like creatures, continuing to head down until they break through the crust and end up in a subterranean world.

As Searcher and Legend are separated from the group during the crash, they get attacked by a creature called a Reaper only to be rescued by Jaeger who has been living underground all these years, revealing that he has tried taking advantage of traversing under the mountain, but is blocked off by an acidic ocean that he cannot cross. He decides to help look for the Venture as a means to cross it. Meanwhile, the group tries to prepare the ship, but Ethan sneaks away to find Searcher and discovers a blue creature he dubs Splat, who tries taking him to his father. They end up getting attacked by more Reapers, but reunite with Searcher, Legend and Jaeger. They are soon rescued by Meridian and Callisto who outrun the creatures and return to the Venture.