In Getting More Than You Battled For, Ash sent Lucario out to battle Mewtwo on Cero Island, teaming up with Goh's Cinderace after Pikachu had been defeated. However, despite putting in its best effort, Lucario stood no chance against the Genetic Pokémon and was defeated by a massive Shadow Ball.
In The Gates of Warp, Lucario, along with Inteleon, was stolen and taken away through a portal by Alternate World Team Rocket. In Showdown at the Gates of Warp, Lucario devolved back into Riolu and finally an Egg as a result of a clash between Dialga and Palkia distorting time. After Arceus stopped the fight between Dialga and Palkia, the Temporal and Spatial Pokémon repaired the distortions their fighting had caused and restored all the devolved Pokémon back to normal, with Lucario returning to Ash's team in the process.
In Battling in the Freezing Raid, Lucario was Ash's second Pokémon in a Raid Battle against an Articuno. Although it managed to land some strong hits on Articuno, it was ultimately defeated by a powerful Blizzard.
In Friends, Rivals, Lend Me Your Spirit, Ash visited Pallet Town and introduced his Pokémon to his new friends that he caught during his journey around the world, including Lucario. During the visit, Lucario trained with Heracross, Buizel, Berloom, Ninjask and Hawlucha. Ash then used Lucario as his first Pokémon in his battle against Paul, using it against his Gyarados. During the battle, Lucario learned Bullet Punch, and was able to defeat Gyarados. Before Ash left his hometown, it posed with the rest of Ash's Pokémon in a group.