Well, this is an interesting way to find out what a movie is about. Sonic the Hedgehog 2 is currently in production, and we know that both Knuckles and Tails will appear in it. Now, though, it looks like we know the plot of the film thanks to a government copyright listing for the project.

The synopsis, which was first noticed by Twitter user @Ninja_Risu, reads, "After settling in Green Hills, Sonic is ready for more freedom, and Tom and Maddie agree to leave him home while they go on vacation. But, no sooner are they gone, when Dr. Robotnik comes back, this time with a new partner, Knuckles, in search for an emerald that has the power to both build and destroy civilizations. Sonic teams up with his own sidekick, Tails, and together they embark on a journey to find the emerald before it falls into the wrong hands."

What we can take away from this is that Knuckles is a villain, Tails is, of course, Sonic's friend to the end, and it looks like Chaos Emeralds are being introduced. Also, while Jim Carrey's Robotnik likely plays a big role, Sonic's human friend Tom (James Marsden) might be out of town for the majority of the film.