Ash's Chimchar evolved into Monferno during a full battle against
Paul just after it defeated
Paul's Ursaring. Having grown in strength and power due to its evolution, it quickly showed everyone that it was a very formidable opponent. It learned Mach Punch right after its evolution from a Chimchar.
Ash's Monferno was
Ash Ketchum's first choice for his battle against
Barry in the town near Sunyshore City. During the battle,
Barry's Empoleon launched a powerful Hydro Cannon attack which sent Monferno flying into the nearby forest. As
Ash Ketchum was calling out to it, Team Rocket suddenly appeared and managed to trap
Ash Ketchum and the others, as well as capture
Ash's Pikachu,
Dawn's Piplup and
Barry's Empoleon shortly afterwards. Upon Monferno's return it was revealed that its ability, Blaze, had activated as it had taken a lot of damage from Empoleon's Hydro Cannon, and that led to it not recognizing
Ash Ketchum. Team Rocket tried to take advantage of the situation, but
Ash's Monferno managed to escape and, after attacking and eventually destroying their mecha, it sent them blasting off.
Then, in order to prevent the mecha from collapsing while
Ash's Pikachu,
Dawn's Piplup and
Barry's Empoleon were still trapped into its container,
Paul sent out his Electabuzz and ordered it to use Protect. In the meantime,
Ash Ketchum tried to calm his Monferno down and help it overcome its rage that was caused by the activation of Blaze. Just as Electabuzz's Protect started cracking under the weight of Team Rocket's mecha, Monferno snapped out of its fury and rushed to help the trapped Pokémon. During the process, it evolved into an Infernape. After
Ash's Pikachu and the others were saved,
Paul told
Ash Ketchum that Monferno had turned the power of Blaze into energy for evolution, which meant that it had finally managed to control its ability.