Pokémon, those little battling, babbling critters of various forms, have been around for what seems like forever, in video games, TV series and feature-length cartoons. But this is the first time they have moved into the current live-action CGI blockbuster format. They would have been better off declining the invitation to do so.

“Pokémon Detective Pikachu” continues the departure from the old franchise formula by being based not on the original games but on the “Detective Pikachu” one issued in 2016, which introduced the furry yellow fellow with a lightning tail as a shamus who can actually converse with a human—young Tom Goodman.

The adaptation by no fewer than five writers (one, Nicole Perlman, receiving only “story by” credit) explains Tim’s (Justice Smith) background: an insurance assessor, he’s long been estranged from his detective dad Harry, who has been working for years in Ryme City, a sort of peaceful urban paradise where Pokémon and humans live together in harmony. The place is the fabulous creation of mogul Howard Clifford (Bill Nighy), who has unfortunately suffered an injury that has left him wheelchair-bound, and whose nasty son Roger (Chris Geere) has taken over the business.

Informed that his father has died in a terrible car crash, Tim travels to Ryme City, where he quickly meets not only his father’s cop friend (Ken Watanabe) but news channel intern Lucy (Kathryn Newton), who’s searching for leads in a story about Harry. Most importantly, he encounters Pikachu (voiced, in “Deadpool” mode, by Ryan Reynolds), Harry’s erstwhile partner, who was in the car with him when it crashed but somehow escaped, though his memory was wiped in the process. Tim reluctantly joins forces with the fast-talking little fellow to try to find out whether his father might still be alive too.

That leads to an avalanche of CGI action set-pieces, involving scads of various Pokémon that those who have played the games and collected the trading cards over the years will probably recognize, but non-adherents will simply find the sort of mostly anonymous special-effects critters, some cute and some not, they’ve encountered in similar live action-animated mash-ups like the “Teenage Mutant Ninja Turtle” movies, the nearly-forgotten “R.I.P.D.” (one of Reynolds’ earlier bombs), or director Rob Letterman’s previous mediocrities, “Gulliver’s Travels” and “Goosebumps.” And however cute they might be initially, they are inevitably affected by another plot ingredient—a green mist called “R,” which turns them into rampaging beasts. Who is manufacturing it, and why, are questions that Tim and Pikachu try to answer to solve the puzzle of Harry’s fate, but the truth will be all too obvious to anybody with half a brain very quickly.

As the plot rolls inexorably—and tediously—on, the movie becomes more and more a cluttered, overstuffed CGI mess. At one point Tim and Pikachu have to flee a gaggle of ravenous Pokémon affected by “R”—and Tim loses his pants in the process (ha, ha!); then Pikachu has to do battle in a cage fight against a huge Pokémon opponent; then they and Lucy get trapped in a terrible earthquake which turns into the movie’s E.T. moment (you know, Pikachu is injured and must be revived); and it all leads to a big finale in which the villain threatens humanity during a parade through the streets of Ryme City and our heroes must unmask him and save the day.

There is one segment in all the chaos that stands out as moderately enjoyable—an encounter with a mime Pokémon. But that’s a throwaway. Devotees of the property will probably also enjoy the presence of MewTwo, who becomes an integral part of the solution to the mystery.

As a whole, though, while “Detective Pikachu” might please fans of the franchise, it will likely leave newcomers cold. It would certainly have helped if Smith were a more ingratiating fellow, or Reynolds had been provided with better lines; he really doesn’t have much to deal with. And as with so many movies of this ilk, the CGI is adequate without being exceptional (the figure of Pikachu excepted), and the action scenes aren’t especially well choreographed, shot or edited (the cinematography is by John Mathieson, the editing by Mark Sanger and James Thomas).

“Detective Pikachu” has the feel of a tentpole start-up, and perhaps with help from the international audience it will prove the start of a series. If so, however, it will be because it feeds into the love of nostalgia that the “Transformers” movies fed into rather than any innate imaginativeness or cleverness. It’s not as bad as the Michael Bay movies, to be sure, but it’s not good either.