"Pokémon - Pikachu's PikaBoo" is a short anime special that centers on Ash, Pikachu, and their friends as they encounter a mischievous ghost Pokémon, Mimikyu, during a festive Halloween celebration. The story unfolds in a spooky forest where the group stumbles upon a mysterious mansion. Inside, they discover that Mimikyu has been playing pranks on visitors, using its ghostly abilities to create illusions and scare them. The mansion is filled with eerie decorations and ghostly apparitions, adding to the Halloween atmosphere.

As Ash and his friends explore the mansion, they encounter various ghost-type Pokémon, including Gengar and Haunter, who are also part of Mimikyu's pranks. The group becomes separated, and each character faces their own fears and challenges. Pikachu, in particular, is targeted by Mimikyu, which has a deep-seated jealousy of Pikachu's popularity and seeks to outshine it. Mimikyu's antics escalate, creating increasingly elaborate illusions to confuse and frighten the group.

The narrative builds as Ash and his friends work together to uncover the truth behind Mimikyu's behavior. They learn that Mimikyu's pranks stem from its desire for attention and recognition, as it feels overshadowed by Pikachu. Through teamwork and understanding, Ash and Pikachu manage to reach out to Mimikyu, helping it realize that it doesn't need to compete with others to be valued. The special concludes with a heartwarming resolution, as Mimikyu joins the group for a festive Halloween celebration, embracing its unique identity.

The main characters include Ash Ketchum, the determined Pokémon Trainer; Pikachu, his loyal and iconic partner; and their friends, who provide support and camaraderie throughout the adventure. Mimikyu serves as the central antagonist-turned-ally, driving the plot with its mischievous yet sympathetic motivations. The special blends humor, suspense, and themes of friendship and self-acceptance, making it a memorable addition to the Pokémon anime series.