The movie was first revealed in July 2016. Filming took place in London from January to May 2018. The logo and title for were revealed during the opening ceremony of the 2018 World Championships on August 24, 2018.
The story begins when ace private eye Harry Goodman goes mysteriously missing, prompting his 21-year-old son Tim to find out what happened. Aiding in the investigation is Harry’s former Pokémon partner, Detective Pikachu: a hilariously wise-cracking, adorable super-sleuth who is a puzzlement even to himself. Finding that they are uniquely equipped to communicate with one another, Tim and Pikachu join forces on a thrilling adventure to unravel the tangled mystery. Chasing clues together through the neon-lit streets of Ryme City—a sprawling, modern metropolis where humans and Pokémon live side by side in a hyper-realistic live-action world—they encounter a diverse cast of Pokémon characters and uncover a shocking plot that could destroy this peaceful co-existence and threaten the whole Pokémon universe.
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After his father, Harry Goodman, disappears in a past automobile accident, Tim Goodman travels to Ryme City to look for clues, and meets the gruff, talkative Detective Pikachu, who was partnered with Tim's father. They team up to search for him, along with solving other mysteries across Ryme City. Along the way, they encounter Lucy Stevens, an up-and-coming reporter who assists them in their attempt to find out what happened to Harry. The investigation leads them to discover the existence of the dangerous R gas, which strips away a Pokémon's higher cognitive functions and reduces them to mindless rage. It is eventually revealed that the true villain of the story is Howard Clifford, the founder of Ryme City, who wanted to capture Mewtwo so as to use its powers to merge Pokémon and people together. It is revealed that Harry was working with Clifford to help capture Mewtwo, but ultimately betrayed him due to moral reservations about the project. Though Clifford attempted to murder him using his squadron of Greninja, a grateful Mewtwo saved his life by merging his soul with that of his partner Pikachu and sending him to find his son. After defeating Clifford and saving Mewtwo, as well as the population of Ryme City, Mewtwo reverses the merger and Harry and his son finally reconcile. Tim decides to stay in Ryme City to live with his father and his Pikachu, now an ordinary Pikachu once again.
The gangster movie playing on the TV when Tim enters Harry's apartment for the first time is the movie that Kevin McCallister watches in the 1991 film Home Alone. The movie in question, titled "Angels with Filthy Souls", is not a real one, and was made specially for use in Home Alone.
When attempting to calm Psyduck, Detective Pikachu yells "Serenity Now!". This is a reference to the 1997 Seinfeld episode "The Serenity Now", in which Frank Costanza is told to yell the phrase whenever he is angry, in order to lower his blood pressure.
This is the first Pokémon product to be dubbed into Flemish Dutch. All previous Pokémon products that were released in Dutch in Belgium used the dub from the Netherlands instead.