The Main Storyline of Detective Pikachu takes place two months after Harry Goodman mysteriously disappeared after being in a car accident, with Tim Goodman, the son of the detective himself, arriving in Ryme City. Tim is there to Find out exactly what happened to his father, receiving the help of Mike Baker at the Baker Detective Agency in order to do this very thing.
Almost immediately after arriving in Ryme City, Tim witnesses an Aipom suddenly steal the Necklace of a young Girl. While trying to stop the Aipom, Tim will have his town map stolen in the process of being accosted as well. During said Attack, Tim will meet and be aided by Pikachu, the same that use to be the former partner of Harry Goodman. Unfortunately, Pikachu lost its memory after the same accident that saw Harry suddenly disappear, not remember practically anything that the two discovered as a team.
After officially meeting, Tim and Pikachu will agree to stop Aipom together, to solve the Case of the missing Necklace. Once completing this very task, the duo will remain a team for the remainder of the game, growing an especially strong friendship in the process. Case by case, Tim and Pikachu will work together, hoping to ultimately Find out what happened to Harry Goodman. In the process of doing so, however, the team will stumble upon a case so diabolical and huge that not even the great Detective of Harry Goodman could solve.