Kasumi, the Cerulean City Gym Leader specializing in Water-type Pokémon, receives an invitation to Dr. Yung's laboratory alongside Satoshi and Professor Okido. During a demonstration of the Mirage System, she battles a Mirage Aggron using her Staryu. Despite unleashing Staryu's most powerful attacks, the Mirage Aggron remains completely unaffected, swiftly defeating Staryu.

After the system malfunctions and the Mirage Master abducts Professor Okido and Dr. Yung, Kasumi helps formulate an infiltration strategy. She teams up with Satoshi to navigate the fortress moat underwater, utilizing her Staryu and Satoshi's Corphish to gain entry. Inside the facility, Kasumi encounters Roketto-dan. She, Satoshi, and Pikachu are then captured by Mirage Machamp and Mirage Ursaring.

During an escape attempt within the fortress, Kasumi loses her grip on a bridge, clinging precariously to the edge. She vocalizes frustration about her status as the Cerulean City Gym Leader. Haruka intervenes, rescuing Kasumi from the fall. After regrouping with the others outside the fortress, Kasumi witnesses the creation of Mirage Mewtwo and the Mirage System's expansion. She observes the battle against Mirage Mewtwo and Dr. Yung, including Mew's intervention and the Mirage System's eventual destruction.

Information regarding Kasumi's background story, development, or additional appearances in other official media beyond this special is not provided in the available sources.