Slakoth lived in Norman's greenhouse when it wasn't being used in battle. The heroes, along with Kenny, visited Slakoth. May, Max, Brock and Kenny left Ash, since they ran off to deal with Norman, who was seen at the Pokémon Center. Once Team Rocket tried to steal Norman's Pokémon, Ash ran off with Vigoroth and Slakoth out of the greenhouse.
It was used in Norman's Gym battle with Ash Ketchum. It first battled Ash's Pikachu, who started off with Quick Attack. Despite its Truant ability, Slakoth evaded the attack and retaliated with Shadow Ball. It was also shown to be flexible, since it was capable of leaning back to evade Pikachu's Iron Tail, then use Hidden Power to negate Pikachu's Thunderbolt and damage him further. Pikachu attempted to use Thunder, but Slakoth used Blizzard to negate the attack and hurt Pikachu. Instead, Ash swapped Pikachu with Torkoal. Slakoth was no match for Ash's Torkoal and despite using Blizzard, Slakoth was defeated by a single Flamethrower.
Before arriving to Slateport City for May's Grand Festival, she and Ash remembered their past adventures so far. Ash recalled facing Norman and his Slakoth, whose uncanny dodge evaded Pikachu's Quick Attack and Iron Tail. Norman and his Pokémon, including Slakoth, watched the TV broadcast of May's Grand Festival battle in his Gym.