Battling the Enemy Within! (Japanese: バトルピラミッド！ＶＳレジロック！！ Battle Pyramid! VS Regirock!!) is the 33rd episode of Pokémon: Battle Frontier.
After a long journey, Ash and his friends have ruined at the Battle Pyramid but things turn into a disaster when they fall into some ancient ruins with a man named Brandon who possesses a Regirock ultimately saving them from death. However, what should be a happy reunion soon turns ugly when Ash becomes possessed by the evil spirit of the King of the Ancient Empire Pokélantis. With no options left, Brandon and the King of Pokelantis engage in a one-on-one battle but this isn't an ordinary battle: it's a battle where the fate of the entire world hangs in the balance. Can Brandon succeed, freeing Ash from the evil King's control or will the entire Pokémon world fall victim to the evil King's control?
Ash is skeptical, as Max leads him to a forest where the Battle Pyramid should be. His doubts are removed as they see a large pyramid. May reminds him when he wins, they go to Indigo Plateau. Ash tells her he will win in no time, but the heroes fall down into a deep hole! Later, however, everyone is more or less okay. Max wonders if Team Rocket dug the hole, but Brock tells him unlikely, as they are in some sort of ruins. To get out, they see some light, so they go to there. Team Rocket see the huge hole and suspect the twerps fell into it. However, the size of the hole increases and they fall in as well.
The heroes see a statue of Pikachu. Max warns Ash not to take it, but Ash just goes to see it, triggering something, as the eyes of statue of Pikachu glow. A nearby door opens and a large boulder rolls down, so the heroes run away. A man sees them and knows they activated the trap, so he sends his Regirock. Regirock uses Forcus Punch to destroy the boulder. Ash thanks the man for saving their lives, but the man yells at them, as his name is Brandon. The heroes tell the ground collapsed and they wanted to get out and, well, Ash touched the Pikachu's statue. Brandon recognizes Ash as the Pokémon trainer and tells him he should be ashamed - a trainer should be on his guard at all times. Brandon tells that the ruins they are in belongs to a nation who wanted to conquer the planet by using Ho-Oh, and that nation is Pokétlantis. Ho-Oh rebelled and the nation was defeated, but the king supposedly escaped and took revenge by sealing Ho-Oh and placed it in an orb and dug the ground to place it.
Team Rocket hears this and want to get that orb to free Ho-Oh and run find it. Ash would like to see the orb, but Brandon does not want "kids" to wander in the ruins. Ash argues, as he got the six Frontier Symbols, and runs to see that orb. Later, Jessie takes a statue of Smoochum, though the trap activates and they are being chased by the boulder. Ash comes to a passage and touches, it, but it is a hidden door. May and Max try to open the door, but cannot. Brandon tells he must be in the King's chamber, and need to find the mechanism to open the door. Ash and Pikachu see the throne and Ash takes an orb. Brandon found the button and presses it, so May, Max, Brock and Brandon go inside the chamber. Ash tells "Ho-Oh, come out here!" and the orb glows, releasing a dark energy to Ash. Ash stands up, but the spirit possessed him.
Team Rocket came to another door. Meowth spots through the lock there is something inside with wings, thinking it is Ho-Oh. However, when they open it, Team Rocket are being chased by a Pidgey. Meanwhile, Ash (with a different voice) tells to hand over the Pokémon, and the dark energy takes May's Poké Balls, but Max and May take them back. Max, May and Brock demand to know why Ash is acting like that, but Ash tells them why are they talking to the King of Pokélantis like that. Brandon tells the heroes Ash is possessed by the spirit of Pokélantis. The King tells them he will conquer the world once more. Brandon tells him the evil was what it made him defeated and Brandon will defeat him once more. Brandon tells the King he is also the King - the Frontier Brain, Brandon the Pyramid King. If Brandon loses, the world will be the spirit's and the Brandon's Pokémon his own, so the spirit accepts the challenge.
Team Rocket see a door, but it is bashed. Meowth yells for Ho-Oh and they hear a mighty roar, so they go inside. However, the Pokémon is Salamance, who makes them being chased by a Flamethrower. Scott comes and greets everyone, telling Brandon Ash is one tough opponent, even if Brandon thinks Ash is still a foolish child. The spirit controlling Ash pushes Scott away to get to the battle field. The heroes tell Scott what happened. Brock tells Scott Brandon knows a lot about ruins, so Scott says Brandon divides his time on doing research on Pokémon and on ruins. Scott would like to know what Ash has under his sleeve to defeat Brandon. The spirit and Brandon will use one Pokémon each. Brandon sends Regirock. Ash wakes up inside his body and realizes he has a battle. The spirit sends Sceptile, so the heroes believe Ash must be inside. Ash declares Sceptile to use Leaf Blade, so the spirit orders him to use it, but Regirock uses Iron Defense for protection.
Sceptile uses Bullet Seed, though Regirock uses Iron Defense. The spirit declares Sceptile to use Bullet Seed on the ceiling, so he does, bringing boulders to hit Regirock. Regirock uses Iron Defense to protect itself, though the stones trap Regirock. Brandon calls the spirit a coward, so Ash hears this and regains the control of his body. However, the spirit still takes the control. Also, Regirock comes out from the stones and uses Hyper Beam, so the spirit orders Sceptile to go behind the referee. Sceptile hesitates, but does anyway, so Regirock is forced to stop the attack. Sceptile uses Leaf Blade on Regirock's leg, wounding it. The spirit taunts Brandon, but Regirock uses Lock-On. Sceptile uses Leaf Blade, wounding the other leg. Regirock uses Hyper Beam and attacks. Though Sceptile jumped to evade, because of Lock-On, the attack is redirected and Sceptile is hit and falls to the ground.
The referee declares the battle over, as Sceptile was defeated. The spirit tells it is not over yet and wants Pikachu to battle. However, Brock, Max and May tell Ash that he should battle the spirit. Ash does not know what to do and cries for help to his friends, but the spirit orders Pikachu to use Thunderbolt. Ash is convinced to protect his friends, so orders Pikachu to use Thunderbolt on him. Pikachu uses Thunderbolt, zapping Ash, so the spirit leaves his body. Brandon waves the orb and says "Light come forth from the dark and return to the dark! Dark come forth from the light and return to the dark!" The orb glows and the spirit is trapped inside the orb once more. Pikachu, Max, May and Brock come to Ash. Brandon says to Scott he "pushed" Ash to come forth, as his friends were the main reason the spirit was defeated. Team Rocket open a door, but it is a Hoothoot. Jessie throws a rock in disappointment, angering Hoothoot enough to attack them. They step on another trap and they are blasted off.
Ash thanks his friends for bringing him back. However, Brandon corrects them and says Ash was defeated by himself - the spirit possessed him because Ash was reckless. Ash tells him he might be a foolish child, but still wants to battle Brandon in an official battle. However, the referee comes to tell Brandon it is time to move. Brandon asked Ash that he cannot battle him, but another time he will be able to and gives advice he should train until that day. Brandon and the referee go inside the Battle Pyramid and the pyramid flies up, revealing to be a plane as well. Ash promises Brandon that he will train hard for the battle. Suddenly, everyone sees a Ho-Oh and recalls that when they started the journey, Ash and Pikachu saw it. So, Ash will train for the battle, while May goes to compete in the Grand Festival.
- Pokémon Symphonic Medley is used as background music.
- An arrangement of Route 27's music can be heard at the end of the episode.
- The Pokémon Red and Blue opening theme is used as background music for the next episode preview.
- The "giant boulder rolling down a narrow corridor" scene from the first Indiana Jones movie, Raiders of the Lost Ark, is reenacted twice in this episode, with Pikachu and Smoochum statues in place of the fertility idol.
- A similar scene would occur in Explorers of the Hero's Ruin!.
- The ongoing Japanese pun in this episode was to have Pokémon appear to Team Rocket that have similar names to Houou. These Pokémon included Poppo, Hoho, and Bohmander. This joke could not be translated into the dubs since most of these Pokémon have names that sound nothing like Ho-Oh's name in each language.
- One of the doors in the ruins appears similar to the wall where the four treasures were placed in A Ruin with a View.
- This is the first time that Ash and Pikachu are not alone when they see Ho-Oh.
- May narrates the preview for the next episode.
- This episode marks the first time that Ash is possessed by an evil entity. The second time it will happen to him will be in Hoopa and the Clash of Ages.
During Ash's flashback Ho-oh has its normal coloration which is different to the gold colored version of Ho-oh shown in the first episode.