Ever since the first Detective Pikachu movie trailer dropped, the film has focused on the fact that its version of Pikachu could talk. Pikachu talking is a driving force of the film's plot, as it allows him to communicate with his partner Tim Goodman, in turn giving the two of them the ability to investigate the mystery behind the death of Tim's father, Harry Goodman. Since most Pokemon can't talk, it's understandable some have been wondering why Detective Pikachu can talk in the movie. As expected, the film explains why, but to discuss it we are going to have to touch on MAJOR SPOILERS for Detective Pikachu, so proceed with caution.
Early in the Detective Pikachu film, Tim is investigating his father's apartment. It's here that Tim discovers a bedroom made to look like his one back at his childhood home, the titular Detective Pikachu, and some mysterious vials marked "R." We learn later in the film that these "R" vials are infused with some kind of special gas extracted from Mewtwo that, when exposed to Pokemon, turns them into vicious, mindless beasts.
Tim accidentally inhales the gas, and it's implied that is part of the reason why he is able to talk to and understand Detective Pikachu. However, Tim is unable to communicate with any other Pokemon in the film, and while that may seem inconsistent, there is an explanation for why that is.
Part of the reason why Detective Pikachu can talk to Tim (and why Detective Pikachu sounds like Ryan Reynolds), is that Tim's dad Harry is Pikachu. This twist is revealed at the end of the movie, when we discover that Mewtwo placed Harry's soul inside Pikachu. This process gives Detective Pikachu a bit of amnesia, so he doesn't remember his real identity for most of the movie.
The concept of combining human and Pokemon souls is seen elsewhere in the Detective Pikachu film as well. In fact, it is the goal of the movie's main villain, so this twist is somewhat foreshadowed before it actually comes to light.
While the mystery of why Detective Pikachu can talk in the Detective Pikachu movie is answered, the film leaves some other things a bit open-ended to perhaps be followed up on in a future film (like the massive Torterra, for instance). Whether or not that happens will largely depend on Detective Pikachu's box office performance, however.
Detective Pikachu is in theaters now.