Caring for a Mystery!.While out searching for Pokemon eggs, Ash and Goh encounter Hayden, a Trainer who challenges Ash to a World Coronation Series ranking match, but his Tauros is defeated by Pikachu. At the Vermillion City Pokemon Center, while Pikachu is being treated, a Pokemon egg from the Sinnoh Region that hasn't been willing to hatch yet reacts to Ash, and Nurse Joy entrusts the egg to him. That night at the Cerise Institute, the egg hatches to reveal a Riolu, who runs away and starts attacking wild Pokemon indiscriminately, including an Onix that may be too strong for Riolu unless Ash and Pikachu can get there in time.