Many walls in the Reflection Cave are reflective like mirrors, thus explaining the origin of the cave's name. Due to these reflective surfaces, several Trainers facing the walls will be able to notice the player walking behind them and challenge them to a battle. The player can also use these surfaces to locate a hidden passageway on B2F. A Pokémon Breeder on B1F will fully heal the player's Pokémon if she's talked to.
The Reflection Cave appeared in The Cave of Mirrors!. Like in the games, it is filled with many reflective crystals. While traveling through the cave, Ash's mirror image suddenly seemed to come alive and grabbed Pikachu, prompting him to follow. He soon found out that on the other side of the Reflection Cave's mirrors, there was a Mirror World, where he met mirror versions of himself and his friends, all of whom possessed characteristics completely opposite to their real world counterparts: Mirror Ash was timid and easily shaken; Pikachu was mischievous and a roughneck; Mirror Serena was arrogant and a serious battler; Mirror Clemont was a fit and fast runner, who relied on magic instead of science; Mirror Bonnie was lady-like and proud of her big brother's achievements; and Mirror Team Rocket were celebrated heroes of justice.
The real Ash soon learned that after losing a battle against Mirror Serena, Mirror Ash tearfully ran into the Mirror World's Reflection Cave, where he became separated with his Pikachu. After seeing the real Ash's Pikachu in a mirror, he reached out and grabbed it, believing it to be his own. After being reunited, Ash and Pikachu agreed to help find Pikachu's counterpart, which turned out to have been caught by Team Rocket, also having found their way into the Mirror World. As they began to attack the real Pikachu, both Ashes sent their Hawlucha to battle them, but the real Hawlucha soon became confused by James's Inkay's Psybeam, and was quickly recalled before it started hurting its counterpart. Mirror Ash's Hawlucha was quickly called back as well, being too cowardly and scared to fight Jessie's Pumpkaboo effectively. However, during the commotion, Mirror Clemont sneaked behind the trio and freed the two Pikachu, who proceeded to send Team Rocket blasting off with a double Thunderbolt.
Afterwards, the real Ash managed to re-open the portal between the real world and Mirror World, and barely managed to return back to his friends before the portal was closed again, his friends and their Mirror World counterparts wishing each other good luck on their journeys. Ash and his friends then exited the cave, deciding to find a Pokémon Center to spend their night in. The episode ended with the Team Rocket trio wandering in the Mirror World forest before being found by their heroic counterparts.