At some point prior to the events of the movie, Sebastian was interrogated by Harry Goodman and Detective Pikachu while searching for the source of the R chemical, which led to a battle during which Pikachu's Electric shocks partially tore Sebastian's prized jacket and scarred his Charizard.
During the movie, Harry's son Tim and Detective Pikachu encountered Sebastian while searching for clues relating to Harry's disappearance, but Sebastian prompted them to have a rematch. They agreed, but Sebastian drugged his Charizard with R, causing it to try and violently attack Detective Pikachu. It was stopped by Tim, who stepped on its tail. Sebastian stepped into the ring to try and stop him, but Tim tackled him to the ground to save him from Charizard's rampage. This released the large amounts of R stored in Sebastian's jacket, which sent all of the Pokémon in the stadium into a frenzy. Sebastian then admitted to Tim that he received the R substance from Dr. Ann Laurent, then escaped amidst the chaos.