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

In the same crazy way that 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 also his long-lost insane grandfather Gerald.

The funny effect of seeing Carrey vs. Carrey, both with mustaches that you could see from space, is like Dr. Evil and Fat Bastard or the Klumps who make an effort at dinner.

Maybe even crazier when you consider that the Robotniks’ enemies include a hedgehog, a fox, an anteater, and James Marsden.

Sonic and Co. are all but retired when a military group called GUN begs them to stop Shadow, a hedgehog alien more powerful than the super-fast Sonic, who has escaped from his cell in Tokyo.

GUN believes the spiky pest is on the hunt for the two keys (yes, it’s another MacGuffin) to a device that can destroy the world.

Robotnik and Gramps also want to grab them for their own nefarious purposes – and suddenly Sonic’s arch enemy is on his side. Somehow.

In this case, Sonic, Knuckles and Tails, along with humans Tom (Marsden) and Maddie (Tika Sumpter), travel 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 more than annoying thanks to the addition of a squeaky fox named Tails (actress Colleen O’Shaughnessey also appears in “Digimon”). For the most part, the voice acting is still of the Saturday morning cartoon variety.

The difference from the previous entry is that despite a formulaic plot, the humor here is balanced with surprising dramatic power.

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

“Shadow” is voiced by Keanu Reeves in his trademark deep voice, who is as good at wearing scars as he is at being Ted. This was the first time in the Sonic series that I heard a sentence as serious as, “I’ve felt this pain for so long, that’s all I know.”

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

Sometimes, amusingly, I admit it.

At one point, a little girl asks Sonic’s friend Tails, “Are you Detective Pikachu?” I admit that it’s essentially the same movie. And her stone-faced buddy Knuckles (Idris Elba) is only afraid of one thing: “Casper, the friendly ghost.” Elba’s best vocal performance of all.

Most of the movie consists of non-stop silly scenes like Sonic and the other happy action figures.

But as fast as Sonic is, it’s Carrey who runs away with the film.