Harley and Cacturne met up with May, who scanned Cacturne with her Pokédex. She noted how scary it looked, despite just standing around. Harley and Cacturne participated in the Izabe Island Pokémon Contest, where Cacturne used Bullet Seed. This formed a mark of a skull and crossbones, which impressed the judges to let Harley into the next round. Cacturne fought against an Azurill, who used Bubble Beam, followed by Slam. However, the bubbles got pierced by Cactrune's Poison Sting and Azurill was defeated by its Bullet Seed. In the final round, Cacturne faced May's Bulbasaur.
Cacturne started with Poison Sting, but was negated by Bulbasaur's Vine Whip. Bulbasaur also launched Razor Leaf, but was negated with Cacturne's Bullet Seed. It was soon hit by Bulbasaur's Vine Whip and Tackle. After May got embarrassed by the story Max told to Harley, who recorded the conversation and played it, Cacturne kicked Bulbasaur away. Bulbasaur tried to attack with Vine Whip, but got hit by its Poison Sting attack and missed using Razor Leaf. Bulbasaur tried to use Petal Dance as well, but also missed. May, however, managed to cool down and had Bulbasaur stop Cacturne's Feint Attack with Vine Whip. With Petal Dance and Razor Leaf, Cacturne was severely wounded and defeated, making Harley lose the Contest.
Before arriving to Slateport City for May's Grand Festival, she and Ash remembered their past adventures so far. May recalled meeting Harley and his Cacturne, the former being really annoyed by her "lack of knowledge and odd behavior". In the final round of the Contest, Bulbasaur launched Razor Leaf, but missed and got hit by Cacturne's Poison Sting. Cacturne dodged Bulbasaur's Vine Whip and Petal Dance, then proceeded to finish Bulbasaur with Faint Attack. However, Bulbasaur binded Cacturne with Vine Whip and tossed it in midair, making it an easy target for Bulbasaur's Razor Leaf and Petal Dance attacks. Cacturne was defeated, which made May earn her fourth ribbon. For the preliminary round of the Grand Festival, Harley had Cacturne use Bullet Seed. Cacturne did so, displaying its quick speed, and earned 84 points, which was enough to pass the round.
During his battle against May at the Grand Festival, Harley sent Cacturne and Banette against her Beautifly and Bulbasaur. Banette started off with Screech, which lowered Beautifly and Bulbasaur's defenses. In addition, Cacturne fired Bullet Seed on them, which May's Pokémon barely evaded. Bulbasaur used Vine Whip to grasp their legs, and Beautifly emitted Silver Wind to hurt them badly. As Harley's Pokémon stood up, Bulbasaur went to repeat its attack, but Banette's Torment prevented that move from being used. Cacturne fired Pin Missile, but Bulbasaur went atop of Beautifly, and used Petal Dance against Banette's Will-O'-Wisp. Cacturne fired another Pin Missile to counter Petal Dance, but in that distraction, May's Pokémon fired Razor Leaf and Silver Wind. Both of Harley's Pokémon were knocked down, and Harley, due to having no more points left, lost the battle.
Harley also used Cacturne in the Appeal Stage of the Saffron City Contest. It used Cotton Spore, then drilled the spores with Needle Arm, thus earning a victory for that round. To stop May from obtaining the final Ribbon, Harley lent his Banette and Cacturne to Jessie to help her in defeating May. However, Cactune glomped Jessie, having a crush on her. During the first round of the Contest, Jessie had Cacturne slice a carrot with Needle Arm. Next, it emitted Cotton Spore and bursted the spores with Bullet Seed.
Harley used Cacturne in the Kanto Grand Festival in a battle against May where it battled along with Harley's Wigglytuff. It took on her Munchlax and Eevee. Munchlax went to use Focus Punch, along with Eevee's Tackle, to which Wigglytuff inflated itself to bounce off their attacks. As Wigglytuff went to attack, Cacturne used Poison Sting to hit Munchlax and Eevee. Cacturne went to use Needle Arm next, but was quickly defeated with a combination of Focus Punch and Shadow Ball, respectively.
When Drew and May wanted to battle one another, Team Rocket, posing as Contest judges, tried to make their move to steal their Pokémon. Harley saw through their disguises, and sent Cacturne to use Bullet Seed on them, destroying their disguises. Team Rocket later attempted to make their move, but Cacturne blasted them off with Bullet Seed.