While the "current" timeline is pretty much business as usual for our heroes (whatever that means), there is a battle raging in the future to prevent an evil scientist / phys. ed teacher, Mr. Ross (Jason Mantzoukas, The Dictator), from harnessing the awesome destructive power of a Timenado (pretty much what it sounds like) to destroy Earth's entire timeline. In this dark future, we get to see the grizzled, battle-worn versions of the park crew, who have joined forces with the Intergalactic Park Rangers to attempt to stop Ross. If that doesn't sound "Regular Show" enough for you, let me throw in the fact that Rigby (William Salyers) from the future visits Rigby from the current time and tells him he has to stop the high-school version of himself from creating a time machine, because when it blows up, it creates the Timenado that Mr. Ross is using to destroy the timeline. Yup, that sounds about right.
If you like the Regular Show, you'll love Regular Show: The Movie; it's got all of the characters you love, with some cool versions from the future and some less cool versions from the past. I think my favorite alternate form was High Five Ghost from the future, who has an eyepatch, appears to possibly have a robotic hand replacement (a la Star Wars) and commands a towering mech.
If you are interested in the "behind the scenes" stages of bringing a project like this to fruition, you might like it for the special features, alone. The Concept and Movie Art "art galleries" are more accurately described as "slideshows set to background music," but give insight into the variations that were played around with on the way to settling on specific art designs and the final art and the notes that are required to indicate specifics, such as the desired opacities and when things are added for better visualization, but are not to be taken as cues for scale. There is also a video of the original board pitch, where the various scenes are all sketched out on Post-It notes stuck to storyboards and described by J.G. Quintel. There are also two features of animatics, one of deleted animatics and one of animatics from the movie, in the order that they appear in the film. There are a lot of animatics here, representing a good bit of the final film. There are also more expected features, such as a Commentary track and the Movie Trailer.
If you're not already a fan, I feel like you'd probably be a bit lost trying to jump in with The Movie, but if you already know and love Rigby, Mordecai (J.G. Quintel), Benson (Sam Marin), Pops (Sam Marin), Muscle Man (also Sam Marin), Skips (Mark Hamill) and High Five Ghost (J.G. Quintel), then you'll do just fine. Die hard fans should watch for cameos, including: Techmo, Head Baby, and Gene, all of which are helping the IPR in the attempt to thwart Ross. If you're a fan of the show, I highly recommend checking out Regular Show: The Movie.