Riley informed Ash that like his Lucario, he could use Aura. Together they found the source of the problem: A machine operated under Mars's command that was using the Spear Key to create low frequency waves which tortured the Steel-type Pokémon. Lucario attempted to read the Aura coming out of the machine, but at that time, Mars placed the Spear Key into a pillar at the ruins the squadron was excavating. The beam that launched out of the pillar's tip overwhelmed Lucario and caused it to rebel against Riley, even breaking its Poké Ball in half when he tried to recall it.
Even then, however, Lucario wasn't completely under the machine's influence and tried to destroy the machine but couldn't do it. Ash managed to finish the job and destroy the machine and Lucario and the other Steel types were brought back to their senses. When Mars realized her plan was shot, she fled and attempted to blow up Iron Island with bombs, but her efforts were thwarted when both Riley and Lucario used their Auras to contain the blast and send it flying.
Lucario first appears in Grumpy Gliscor along with his Trainer Riley on Iron Island waiting for Diamond to arrive so that they can train him and improve his battling skills. After he arrives they begin their training, Lucario is pitted against Tru, Don, and Lax and with strong moves like Close Combat and Aura Sphere, he puts them into a tight spot before catching Tru's Wood Hammer and holding him while his Aura Sphere's fall from the sky and defeat Diamond's Pokémon. Lucario then oversaw Diamond's Training along with Riley, but during the training his aura sensors pick up something, it was later revealed to be two Cue Ball's who have stolen a Pokémon Egg. The Cue balls blind Riley and Lucario with Vespiquen honey and prevent them from using their Aura with a Mothim's Air Slash, they are defeated by Dia's newest technique the "Mach-one-and-only-Razor-Leaf" and are left to suffer the wrath of Lucario who beats them with Close Combat, Aura Sphere, Bone Rush, and Flash Cannon in quick succession.