Jim Carrey is out of his mind

There really is only one justification for the existence of “Sonic The Hedgehog 3”: Jim Carrey getting the “Austin Powers” treatment.

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

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

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

Sonic and Co. are all but retired when a military group called G.U.N. begs them to stop Shadow, a hedgehog alien more powerful than super-fast Sonic who’s escaped from his Tokyo cell.

G.U.N. believes the prickly pest is hunting down the two keys (yeah, it’s another MacGuffin) to a device capable of destroying the world.

Robotnik and Gramps want to snatch those up, too, for their own nefarious purposes — so, suddenly, Sonic’s nemesis is on his side. Sort of.

On the case, Sonic, Knuckles and Tails, along with humans Tom (Mardsen) and Maddie (Tika Sumpter), journey to Japan, London and the cosmos to thwart a talking porcupine.

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

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

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

Shadow is voiced characteristically deeply by Keanu Reeves, who’s as good at being scarred 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.” 

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

Sometimes, I’ll admit, amusingly so.

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

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

Fast as Sonic is, though, it’s Carrey who runs away with the movie.