Chesnaught was originally Chapman's Pokémon. However, after Chapman died, Chesnaught was ordered to protect his grandson, Henny, as well as Robon, to help them in reviving the land, which withered out.
Seeing the heroes have approached the robot, Robon, Chesnaught descended down from a cliff and started attacking the heroes with Seed Bomb. The heroes ran off, though Chesnaught did not go after them, since it had to protect the robot. Clemont decided to repair Robon, alerting Chesnaught, whom Henny explained the heroes were here to help as well. Since they needed water to revive the land, Chesnaught used Vine Whip to trace water, along with Clemont's Chespin, and found some. After the heroes and Henny found out there was a big rock blocking the access to the water, Chesnaught tried to use Pin Missile on the rock, but failed to destroy it. The heroes sent out their Pokémon to attack the rock, along with Chesnaught, but did not succeed. While the heroes were working on different method, Team Rocket arrived and captured everyone. Jessie sent Gourgeist to attack Robon, which infuriated Chesnaught. Inkay and Gourgeist attacked Chesnaught, who was protected by Robon, who took the hits and destroyed itself. Chesnaught was furious and released itself, then attacked Team Rocket with Pin Missile. This caused Meowth to drop the remote, which freed everyone. Once Team Rocket were blasted off, Henny and Chesnaught cried due to Robon's demise. Fortunately, Clemont managed to repair Robon, pleasing Chesnaught and Henny. In the end, Chesnaught, Robon and Henny bid farewell to the heroes, who moved onwards.