In the vibrant world of Pokémon, Tim Goodman (Justice Smith), a 21-year-old insurance salesman and former Pokémon trainer, is on a quest to find his missing father, Harry (Ryan Reynolds). Haunted by the loss of his mother and his father's absence, Tim has turned his back on Pokémon training. Tragedy strikes when he learns that Harry, a veteran Ryme City police detective, has died in a car accident while working on a case.
Tim travels to the bustling metropolis of Ryme City, a place where Pokémon battles are illegal and humans and Pokémon share a unique bond. Determined to settle his father's affairs, he encounters Lucy Stevens (Kathryn Newton), an ambitious fluff columnist with aspirations of becoming a reporter, who reveals her suspicions surrounding Harry's death. Together with her Pokémon, Psyduck, they delve into the mystery.
In a startling twist, Tim meets a witty, deerstalker-wearing Pikachu (Ryan Reynolds), a remarkable detective whom only Tim can understand. Their confrontation with a group of Aipom, influenced by a mysterious purple gas accidentally released by Tim, propels them into an adventure filled with intrigue. Taking refuge in a café, Pikachu discloses that he was Harry's police partner and has lost his memory, with only a deerstalker hat bearing Harry's name as a clue.
Their investigation leads them to Mr. Mime, a former informant for Harry, who directs them to an underground Pokémon fighting arena owned by Sebastian (Omar Chaparro). Here, Tim confronts Sebastian and his Charizard, who challenge Pikachu to a rematch. In the chaos that ensues, a gas labeled "R" is released, prompting Tim to race against time as the authorities intervene.
Tim's argument with Lieutenant Hideo Yoshida (Ken Watanabe) doesn't yield the support he hoped for when Yoshida presents footage of Harry's accident, casting doubt on the possibility that Harry is still alive. Undeterred, Tim and Pikachu visit the enigmatic benefactor Howard Clifford (Bill Nighy). In a shocking revelation, Howard shows them footage suggesting Harry survived the accident, abducted by the recently escaped Mewtwo, a powerful Pokémon cloned from DNA.
As their investigation deepens, Tim and Pikachu join forces with Lucy and Psyduck to uncover a nefarious plot orchestrated by Dr. Ann Laurent (Rita Ora). They learn that Harry had been aiding in experiments concerning Mewtwo and that trouble is more imminent than they feared. In a thrilling sequence, they evade an onslaught of Greninja in a lab facility but find themselves in a precarious situation with Pikachu suffering grave injuries.
In a dramatic turn of fate, a Bulbasaur leads Tim to a clearing where Mewtwo heals Pikachu. Yet, misunderstanding and betrayal loom as Pikachu leaves Tim, believing he has let Harry down. Meanwhile, a sinister plan unfolds as Howard intends to use Mewtwo to manipulate Pokémon behavior, forcing a fusion of consciousness between humans and their Pokémon.
In a climactic showdown, Tim confronts the truth about Roger (Chris Geere) and the form of Ms. Norman, realizing that deception runs deep within Ryme City. As chaos reigns, Tim manages to thwart Howard's malicious plan, allowing Mewtwo to regain control. The aftermath sees Howard arrested and Roger impressed by Lucy’s determination, culminating in a heartfelt reunion where Mewtwo discloses Harry's fate—his consciousness now residing within Pikachu.
By the film's end, Harry, restored and revived, offers Tim a choice: to return home or stay and pursue his dream of becoming a detective alongside him and Pikachu. Choosing the latter, Tim embraces his new life, filled with adventure and camaraderie in the heart of Ryme City.
