Ash's Infernape was revealed during the Japanese opening 'The Greatest-Everyday!'. It evolved a few episodes after the airing of the opening in DP163.
During a battle against
Barry, Monferno was hit by Empoleon's Hydro Cannon and was sent flying into the nearby forest. As
Ash Ketchum was calling out to it, Team Rocket suddenly appeared and managed to trap him 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.
After snapping out of the rage that was caused by the activation of its ability, Blaze,
Ash's Monferno evolved into an Infernape in order to save
Ash's Pikachu,
Dawn's Piplup and
Barry's Empoleon from Team Rocket's collapsing mecha.
Paul, who had witnessed everything, told
Ash Ketchum that Monferno had managed to control Blaze at last, and that it had turned the power of its ability into energy for evolution. He also informed
Ash Ketchum that his brother wanted to see another full battle between the two of them, and promised to battle against him and Infernape in the Sinnoh League.