Panel 1: Jake and Finn in a crazy time vortex of some sort.
Caption: WHEN WE LAST SAW OUR HEROES*, THE ICE KING HAD, THROUGH A COMBINATION OF SPITE AND INCOMPETENCE, SENT THEM THROUGH A PORTAL INTO A STRANGE AND UNFAMILIAR UNIVERSE!
Caption: *JUST GO WITH ME ON THIS ONE, DUDE.
Panel 2: Jake and Finn are being ejected out of the portal as it closes behind them. It's not clear where they are in this panel, but they're in the house from Regular Show.
Panel 3: Jake and Finn are standing in the house from Regular Show. We can't see the couch, but they're near it. Jake and Finn are confused.
Jake: WHAT IS THIS PLACE, FINN?
Finn: I DUNNO, JAKE, BUT I CAN'T SAY I'VE SEEN ANYTHING LIKE THIS BEFORE.
Panel 4: Same POV, but Jake is running his paws on the walls and Finn is looking around. Both are intrigued.
Jake: MATHEMATICAL, JAKE! WHEREVER WE ARE, IT MUST BE FULL OF STRANGE NEW THINGS TO EXPLORE!
Finn: THE PERFECT PLACE FOR...
Panel 5: Jake and Finn have jumped up for the high five, and the background is what it is on the show's title still.
Jake/Finn in unison(logo): ADVENTURE TI-(this is where it's cut off by the next speech bubble)
Mordecai(OP): UHHHH, HELLO?
Panel 6: We see Mordecai and Rigby on the couch, obviously confused by these guys just appearing in their living room. They'd been playing video games.
Rigby: THE PARK IS OUT THERE DUDES. THIS IS OUR HOUSE.
Mordecai: YOU DO KNOW WHAT A HOUSE IS... RIGHT?