“Dennō Senshi Porygon” was episode 38 of the first Pokémon anime season.
The episode was broadcast in Japan on December 16, 1997. Its story involved Ash and his friends investigating a problem with a Pokémon Center transmission system, with Porygon appearing in the episode’s virtual-reality setting.
During the broadcast, rapidly flashing red and blue imagery caused photosensitive reactions among viewers. More than 600 people were taken to hospitals, and the incident became one of the most widely discussed events in television history involving flashing images.
The episode was withdrawn from regular circulation after its sole Japanese broadcast. A common misconception is that Porygon itself caused the incident or that the episode was simply “banned worldwide”; the reactions were associated with the flashing visual sequence, and the episode was not rebroadcast in Japan.