Mordecai and Rigby are working and they discover a secret abandoned basement that Benson tells them not to go in, but they do it anyway. Then they find a secret machine then aftwards they turn it on, and then a portal appears. Mordecai and Rigby then walk into it and they end up in Ooo.
Having no idea where they are, they are attacked by monsters and creatures and stuff, then afterwards they start arguing that it was the other one's fault. But as they are arguing a tall giant creature shows up and is about to crush them, but luckily Finn and Jake show up, and they all defeat the creature together. After Finn and Jake interface thereselves, Mordecai and Rigby tell them they need help getting back to the portal and getting back home. Ice King then finds the portal and enters Mordecai and Rigby's dimension and plans to take over it. When Marget walks home, she is wearing a crown she got from a party that she just went to, but Ice King mistakes her for a princess then captures her.
After Mordecai and Rigby finally get home, they see that their dimension is taken over by the Ice King and that he captured Margret. The two quickly seek the help of Finn and Jake who kindly agrees to help them (especially if the Ice King is there). Ice King creates a new ice layer and forces Margret to marry her. Margret quickly rejects, but she sees that the Ice King is keeping Eline hostage and threatens to kill her if Margret won't marry him. Margret finally agrees to marry Ice King to save Eline's life. They are close to get married but Finn, Jake, Mordecai, and Rigby break into the ice layer and they all battle the Ice King. Rigby and Jake save Margret and Eline after the Ice King planned to freeze them in solid ice, leaving Finn and Mordecai battling the Ice King. The battle ends with Finn and Mordecai delivering a final punch directly into the Ice King's face, quickly knocking him out.
Then, as the battle is over, Mordecai and Rigby thank Finn and Jake for helping them rescue Margret and Eline and stopping the Ice King from taking over their world. Mordecai and Margret have a moment and almost kisses but doesn't when they see everyone else staring at them. Finn and Jake grabs the unconsious Ice King and then they go back into the portal to the Land of Ooo, and Mordecai and Rigby take Margret and Eline home, ending the story.
The story takes place with Mordecai and Rigby cleaning up the house after throwing another one of their parties, but Benson tells them to clean out the attic. As they are trying to clean the attic, they stumble across a mysterious machine. Benson wondering what is taking so long for them to clean out the attic, comes into the attic himself and yells at them to finish up faster, but Mordecai and Rigby interrupt Benson by saying that they found a strange machine and wants to know what it is but Benson angrility shouts at them to forget about the machine and get back to work. The two then do what Benson ordered them to do and quickly cleans up the attic.
A couple of hours later, Mordecai is shocked to see that Rigby brought the machine into the their room. Mordecai orders Rigby to out the machine back into the attic where it belongs, but Rigby convinces that it would be cool to see what it does. Mordecai still wanting to take the machine back into the attic, Rigby then replies to Mordecai by saying "What are ya chicken?! Buck-Bu ckaww!" causing Mordecai to get annoyed and finally trying to get a way to turn the machine but fails several times. After trying a ton of times, Mordecai gets frustrated after failing so much and kicks the machine finally turning it on all of the sudden. As the machine is on, Mordecai and Rigby walk into the portal as the machine breaks as soon as they walk into it.
The scene then goes to Finn and Jake who are battling the Ice King at the Ice Lair who has just captured Princess Bubblegum. As the two battle the Ice King, Finn sees that the Ice King trapped Princess Bubblegum in a prison cell. "I'll save you, princess!" Finn says while he quickly breaks open the ice prison bars and grabs the princess before having another battle with the Ice King. Ice King angrily demands Finn to hand over Princess, but Finn simply denies by kicking him straight in the jaw. As the battle continues, it ends with Finn and Jake are left victorious while the Ice King is now weak and defenseless.
Before they try leaving the Ice Lair, the Ice King quickly calls upon the gigantic 3-headed Cryo Dragon. Finn quickly gets PB to safety, then joins Jake to battle the dangerous creature. As the battle goes on, one of the dragon heads shoots an ice beam out it's mouth at Jake, freezing him in a block of ice leaving Finn to face the deadly alone. While the Cryo Dragon is victorious, it finally tries to end the battle by using all 3 heads to combine their ice beams into one giant ice beam and releasing it at Finn. Luckily, Finn uses the Family Sword to deflect the giant beam, freezing the Cryo Dragon (ironic ain't it?) and making it fall out the Ice Lair. Finn grabs a flamethrower he got out his backpack and melts that trapped Jake in. Finn and Jake grab PB and escape the Ice Lair while an angered Ice King shouts that he will have his revenge. Finn and Jake drop off PB safely at the Candy Kingdom, as she thanks them both for saving her from the Ice King yet again while all of the sudden, Jake tells Finn that he hears someone calling for help. Finn and Jake tell PB that they'll see her later as they quickly run off to help whoever was in trouble.
The scene moves back to Mordecai and Rigby who are transported to Ooo. Mordecai then shouts at Rigby "Aw, dude now look what you did?!".
"Me?! If it wasn't for you kicking the stupid machine, none of this would've happened!" Rigby shouted back.
While the two are fighting, they are surrounded by a vicious gang known as The Wreckers (Crusher, Devastater, and Scar). "Looks like we get ourselves some fresh meat!" Crusher shouted while swinging his mace.
"Lemme rip out their eyeballs and squeeze the juice out of them to put on my toast!" Devastater also shouts.
"I wanna stomp on their hearts and chew on their kidneys!" Scar replied to Devastater.
Mordecai and Rigby are surrounded by the vicious bloodthirsty gang, as they are shouting "Help!". Luckily Finn and Jake arrive at the scene.
"You Wreckers are about to get wrecked!" Finn and Jake says before Finn kicks Crusher into a tree.
Finn then gets into a sword fight with Scar, then eventually cuts off his arms with his trusty Family Sword. Afterwards Finn grabs Scar's tongue and quickly turns him inside out (which makes Mordecai and Rigby look ill). Jake snatches Devastater's bat and starts beating him with it until it breaks in half, then makes his fist bigger to crush him.
"Hey...where's that Crusher guy?" Jake asked.
"He probably ran off" Finn replies to Jake.
Crusher then sneaks up behind Finn and Jake out of nowhere and tries to attack them from behind, but before he did anything Mordecai and Rigby grab a giant boulder and smashes Crusher's head with it, knocking him out.
"Holy Glob, you guys got skill!" Finn compliments at Mordecai and Rigby.
"Thanks, but where the heck are we?!" Mordecai asks the two.
"Ooo" Jake said.
"Uh...Ooooo what?" Rigby asked looking very confused.
"No man, you're in the land of Ooo!" Jake stated.
"Huh...weird name" Mordecai whispers to Rigby who nods.
"I'm Finn, and this is Jake." Finn says as he intourduces himself to Mordecai and Rigby.
"I'm Mordecai and this is Rigby." Mordecai interduces back.
"So...what do you guys...do here exactly?" Rigby asks Jake.
"We usually have adventures, fight monsters, save princesses. Those kinds of stuff." Jack replies to Rigby.
"Whoa..." Mordecai and Rigby says looking quite amused.
"Yep, we're quite the adventurers. Infact we're the heroes of Ooo!" Finn shouts.
"Whoa, that's cool but-" Mordecai says but is interrupted.
"Infact, you guys don't look like you're from here." Finn interrupts Mordecai.
"That's what I was about to say, dude. We're not from here at all! We're from another universe! We ended up here when he found some machine that transported us to your universe, and now we have no idea how to get back!"
"Well no worries bro, we'll help ya!"
"Really? Thanks, dude!"
Just then, Rigby taps on Mordecai's shoulder and asks to speak with him.
"Huh? Oh hold on a sec you guys" Mordecai says to Rigby.
"Dude, how did a teenager and a dog become heroes of a weird place like this?" Rigby whispers to Mordecai.
"I dunno dude, but he nice. They said they were gonna help us!" Mordecai whispers back to Rigby.
"Dude, how did a blue jay and a raccoon end up in a another universe?" Jake whispers to Finn.
"I dunno bro, but be nice. We agreed that we would help them out." Finn whispers back to Jake.