Mag and Meg, Flannery's other Slugma, accidentally set her hair on fire, while she was training them. When Flannery's Torkoal was stolen by Team Rocket, Flannery sent Mag to attack Team Rocket, negating James' Cacnea's Pin Missile by using Ember. Meowth attempted to attack Max and May, who were rescuing Torkoal, but was burned by Mag's Flamethrower. After stopping Cacnea and Jessie's Seviper by using another Flamethrower, Mag evolved into a Magcargo.
During Flannery's Gym battle with Ash, Mag was the first Pokémon Flannery used. After taking Corphish's Bubble Beam attack, Mag retaliated with Flamethrower, but Corphish used Harden to protect itself. Corphish attacked with Bubble Beam, but missed, due to Mag's Double Team. Instead, Corphish spinned around using Bubble Beam and hit the real Mag. To protect itself, Mag used Reflect, negating Corphish's Vice Grip. Mag used Sandstorm to counter Corphish's Crabhammer. However, Corphish jumped and used Crabhammer, cutting through the sandstorm and defeating Mag.