Detective Pikachu is all about solving a mystery, and as with all good mysteries, the ending comes with a twist! The movie, directed by Rob Letterman, is based on the Nintendo video game of the same name and broadly follows the same plot beats. Tim Goodman (Justice Smith) arrives in Ryme City after hearing about the death of his father in a car accident, only to meet his father's Pikachu partner (Ryan Reynolds), who insists that Harry Goodman is still alive and the car crash was part of a much bigger story.
Tim hasn't seen his father since he was a boy, and isn't interested in having a Pokémon partner like everyone else in Ryme City, but his fuzzy new friend persuades him to join the hunt. Their search for Harry Goodman leads them to business mogul Howard Clifford (Bill Nighy) and his obnoxious son Roger (Chris Geere), as well as the discovery that a powerful man-made Pokémon called Mewtwo escaped on the night of the car crash.
Hampering their investigation is the fact that Detective Pikachu is suffering from amnesia after the crash, and the only clue he had to start with was a hat with Harry's name and address in it. Tim and Pikachu eventually get to the bottom of the mystery: Howard Clifford became obsessed with evolving humanity after falling ill, and began funding experiments on Pokémon to make them bigger, stronger, and more powerful. The pinnacle of this was the creation of Mewtwo from ancient Mew DNA. Mewtwo's DNA was extracted to created the compound R, which causes Pokémon to lose control and - crucially - makes it possible for Mewtwo to put human minds inside Pokémon bodies!
Once Howard Clifford is defeated, Tim and Pikachu finally find Harry Goodman, who is indeed still alive - and in the most unexpected place.
Ryan Reynolds Is Harry Goodman, Who Is Detective Pikachu
In the aftermath of the annual Pokémon Parade, Mewtwo approaches Tim and tells him that the father he's searching for has been with him all along. Yes, Detective Pikachu is Harry Goodman - or rather, Harry's consciousness fused with his Pokémon partner.
Harry Goodman and Detective Pikachu orchestrated Mewtwo's escape from the laboratory, with Pikachu tinkering with the building's electricity to allow Mewtwo to break free. Harry and Pikachu then tried to escape the lab in the car, but genetically enhanced Greninjas chased after them and used throwing stars to blast the car off the road. Fortunately Mewtwo arrived on the scene and fought off the Greninjas, but Harry had already been badly injured. Pikachu offered up his own body to save Harry's life after the crash, and Mewtwo used its powers to fuse the two of them together. The Legendary Pokémon said that while it believed humanity was evil, Harry's actions had proven that some humans were good.
In order to separate Harry Goodman and Detective Pikachu again, Mewtwo first needed Harry's son - presumably because his DNA was necessary to heal Harry in the separation process. Mewtwo was interrupted before it could separate them the first time Tim encountered it, but at the end of the movie Harry and Pikachu's fusion is reversed. In the movie's final scene, we see why Harry's face has been kept concealed: he's actually played by Ryan Reynolds! As he's about to head back, Tim decides that he wants to stay in Ryme City with his father instead, and learn how to become a detective himself.
Detective Pikachu's Twist Confirms a Game Theory
The plot of the Detective Pikachu game plays out similarly to the plot of the movie, with one major difference to the ending: Tim and Detective Pikachu don't find Harry Goodman. While the game also centers around a mysterious compound called R that causes Pokémon to lose control and go on rampages, and the R is derived from Mewtwo's DNA, the drug is not used in the process of merging human minds into Pokémon bodies. Instead, Roger Clifford (who is the main villain in the game) simply wants to use the disaster of rampaging Pokémon to make his news network more powerful.
After defeating Roger, the game ends with Tim and Pikachu continuing on their search for Harry - but fans of the game believed that Tim may have already found his father. Pointing to Detective Pikachu's Dad-ish mannerisms and the fact that Tim is the only person who can understand him, many players concluded that Detective Pikachu was Harry. And as crazy as this theory might sound, the Detective Pikachu movie took it and ran with it.
Human Beings and Pokémon Can Merge
Mewtwo is an extremely powerful Pokémon (just look at Pokémon: The First Movie), but Detective Pikachu gave it a unique power: to merge humans and Pokémon together, so long as the Pokémon are under the influence of the R gas. It's revealed that this was the plan all along, with Howard Clifford recruiting scientist Dr. Ann Laurent (Rita Ora) to experiment with Mew DNA and create a new Pokémon, who would allow humans to "evolve." Meanwhile, Howard himself "evolved" by using a neural link to control Mewtwo, breaking free from the body confined to a wheelchair and wielding the power of a Legendary Pokémon.
There's a definite sense that the reveal of Mewtwo's ability to merge humans and Pokémon was primarily in service of the movie's twist, since there are only so many ways to explain how Harry Goodman could have turned into a Pikachu. Still, this has interesting implications for the future of the movie series (if Detective Pikachu does indeed get a sequel). Mewtwo is still alive and free at the end of the movie, and in theory could merge more humans with Pokémon. Meanwhile, though Harry and his Pikachu partner have been separated and Detective Pikachu is now reduced to saying only "Pika pika," the two could end up fused together again in a future movie.