Mordecai is smitten for Margaret, a redbird who works at his favorite coffee shop. This is a bit of a curve ball for Ribgy, since they used to do everything together, pretty much exclusively. Rigby has other concerns, however, as he'll have to deal with the fact that he, reluctantly, sort of has a thing for Eileen, a short waitress at the Coffee Shop who wears glasses. And both Mordecai and Rigby will have to figure out how to get Muscle Man back together with his girlfriend, Starla, who frighteningly, is a perfect match for Muscle Man.
A good portion of the episodes deal with Mordecai trying to find the perfect opportunity to express his feelings to Margaret. He's not a very smooth operator, and has a really hard time working up to it. This not only wears on Rigby... and Muscle Man... and even Skips, but becomes the basis for a few bets and a good deal of chastising Mordecai.
I haven't been the biggest fan of Regular Show in the past, finding the ridiculousness of the introduction of spontaneous mythos to be a strain to my suspension of disbelief, but something about the desperation of Mordecai for Margaret allows me to accept them as more metaphorical, perhaps, representative of Mordecai's perception of events, rather than the actual events, themselves.
There is just one special feature, but it's amusing: a commercial advertising a fancy steak restaurant featured in the episode, "Steak Me Amadeus." Ridiculous? Yes, but a fun watch.
At any rate, the introduction of girlfriends changes the dynamic of Regular Show quite a bit. As I said, I enjoy the new direction, but diehard fans of the show and its status quo may have different opinions about these changes. Don't worry, however, Regular Show still has the same what-the-heck-just-happened and over-the-top ridiculous premises that you've come to know and love, whether it be a spurned lover going all Incredible Hulk after being dumped or a talking asteroid banishing Mordecai and Margaret into "The Friend Zone" (which is done as a parody of the Phantom Zone from Superman's mythos). If you're looking for your Regular zaniness, it's still in there - only now with a little more romance.