Jim Carrey is beside himself

There’s really only one reason for the existence of “Sonic the Hedgehog 3”: Jim Carrey gets the “Austin Powers” treatment.

In the same crazy way Mike Myers did double and even quadruple duty in his spy comedies, this time Carrey plays not only the over-the-top villain Dr. Robotnik, but also his long-lost mad grandfather Gerald.

The fun effect of watching Carrey vs. Carrey, both with mustaches you could see from outer space, are like Dr. Evil and Fat Bastard or Klumps going at it at noon.

Maybe even crazier, considering Robotniks’ enemies are a hedgehog, fox, anteater and James Marsden.

Sonic and Co. is nearly retired when a military group called GUN asks them to stop Shadow, a hedgehog creature stronger than super-fast Sonic, who has escaped from his Tokyo cell.

GUN believes that the prickly vermin is chasing the two keys (yes, that’s another MacGuffin) to a device capable of destroying the world.

Robotnik and Gramps also want to grab them for their own nefarious purposes – so suddenly Sonic’s nemesis is on his side. Kind of.

In the case, Sonic, Knuckles and Tails along with humans Tom (Marsden) and Maddie (Tika Sumpter) travel to Japan, London and the cosmos to defeat a talking hedgehog.

“Three” is much easier to swallow than the first sequel two years ago, which I found more than annoying thanks to the addition of a squeaky fox named Tails (actress Colleen O’Shaughnessey also appears on “Digimon”). The voice acting is mostly still of the Saturday morning cartoon variety.

What is different from the previous entry is that humor here, despite a formulaic plot, is balanced with surprising dramatic weight.

Strange but effective is Shadow’s tragic backstory. An explosion at a base he lived in caused the death of his only friend, a little girl named Maria. The trauma and the blind rage made him evil.

Shadow is voiced with characteristic depth by Keanu Reeves, who is as good at being Scar as he is at being Ted. This was the first time in the “Sonic” franchise I’ve heard a line as serious as, “I’ve felt this pain for so long, it’s all I know.”

The life-and-death dilemma of director Jeff Fowler’s film adds improbable stakes to a story that would otherwise be, well, extremely stupid.

Sometimes, I’ll admit, funnily enough.

At one point, a little girl asks Sonic’s friend Tails, “Are you Detective Pikachu?” acknowledges that it is essentially the same film. And their stone-faced friend Knuckles (Idris Elba) is only afraid of one thing: “Casper the Friendly Ghost.” Elba is the best vocal performance of the lot.

Most of the movie is non-stop silly bits like that of Sonic and the other yappy action figures.

Fast as Sonic is, however, it is Carrey who runs away with the film.