4x1 – Episode 1
When everyone is brainwashed and the park is destroyed, Mordecai and Rigby have to bring everyone back their old selves so they can prevent a highway from being built over the park.
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4x2 – Episode 2
Mordecai and Rigby try to get Muscleman to stop pranking Thomas when they realize that his pranks are getting out of hand.
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4x3 – Episode 3
Mordecai and Rigby are driving to a Halloween party and tell scary stories to pass the time.
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4x4 – Episode 4
Mordecai and Rigby take over the duty of judging the Park's Pie Contest, but realize that the job isn't all that easy when they find out that they have to pick a winner.
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4x5 – Episode 5
Benson tries to assemble a 150-piece drum kit to prove that he performed a famous drum solo for a well-known rock band.
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4x6 – Episode 6
When Muscle Man notices a bald spot on his head, he tries to cover it to keep Starla from seeing it at their date.
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4x7 – Episode 7
Pops tries to complete the Milk Challenge to prove that he is worthy of becoming one of the guys.
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4x8 – Episode 8
Rigby must perform a single pull-up to avoid being fired on the grounds of poor personal fitness.
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4x9 – Episode 9
Mordecai and Rigby have to destroy a gift for Santa, to save Christmas.
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4x10 – Episode 10
Margaret is transferring to another college, so Mordecai, Rigby and Eileen plan a going-away party at the park's abandoned ballroom, still haunted by its past hosts.
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4x11 – Episode 11
Muscle Man, Mordecai, Rigby, and High five ghost blow up the fourth of July fireworks and are forced to drive to get new ones.
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4x12 – Episode 12
To prove their love for each other, Muscleman and Starla plan to stay away from each other for one weekend.
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4x13 – Episode 13
Mordecai and Rigby buy a potentially deadly sandwich at a restaurant, but when Benson falls ill after he eats it himself, they must set out to find the cure before he dies.
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4x14 – Episode 14
Mordecai and Rigby want to reunite a famous band when they think the lead singer, who was presumably dead, is alive and living in the park.
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4x15 – Episode 15
Muscle Man bets Mordecai that if he doesn't kiss Margaret by Friday at midnight, he will have to wear a diaper for a week.
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4x16 – Episode 16
Skips has to tell is cousin, Quips, to stop telling bad jokes when he comes to visit.
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4x17 – Episode 17
While camping out in a cave, Mordecai and Rigby accidentally melt a caveman frozen in a block of ice. When they try to keep him, Benson requires that they make him more civilized.
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4x18 – Episode 18
Mordecai and Rigby help their favorite '80s sitcom star, RGB2, escape from the studio lot and into the mountains to get out of a contract that has trapped him into staying on the show.
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4x19 – Episode 19
When the park becomes invaded by vicious territorial geese, Mordecai and Rigby seek the help of the baby ducks to get rid of them.
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4x20 – Episode 20
Mordecai and Rigby try to get on a Japanese Game Show so they can win the grand mystery prize.
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4x21 – Episode 21
Mordecai and Rigby accidentally coat the interior of Mr. Maellard's limousine in meatball sub.
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4x22 – Episode 22
When Mordecai volunteers to drive Margaret to the airport, he tries to convince Benson to let him use his car.
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4x23 – Episode 23
Muscleman, Mordecai and Rigby help "Donny G", a famous D.J. take back over the corrupt local radio station so Muscleman can play his song he wrote for Starla on the radio.
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4x24 – Episode 24
Mordecai and Rigby enter a vehicular doughnut contest to win guest spots on their favorite police procedural TV show.
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4x25 – Episode 25
Skips faces dire consequences when he passes up a day off to reduce his deadly amount of stress.
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4x26 – Episode 26
When Thomas is incapacitated by a dangerously cold slushie flavor, Mordecai and Rigby must enter his brain to prevent the drink from killing him.
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4x27 – Episode 27
A health inspector threatens to shut down the park if sanitary conditions inside Muscle Man's trailer are not improved.
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4x28 – Episode 28
Mordecai plans to kiss Margaret at meteor shower to prove to Rigby that he isn't in the "friend zone".
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4x29 – Episode 29
When Mordecai is invited to a BBQ with Margaret's family, he tries to get her father to give him a handshake.
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4x30 – Episode 30
Mordecai and Rigby try to find a laserdisc player so they can see the director's cut of a famous cult classic.
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4x31 – Episode 31
Mordecai and Rigby try to take back the cart when it is stolen by a rich, snobby country club.
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4x32 – Episode 32
Benson has to trust Mordecai and Rigby.
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4x33 – Episode 33
The gang wants to give a Benson a "World's Best Boss" mug.
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4x34 – Episode 34
Muscleman gets the guys to take him to fast food joints so he can binge on his favorite foods one last time before going on a couple-diet with Starla.
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4x35 – Episode 35
The gang has to stop Muscleman from fighting in his sleep by finding the source of his nightmares.
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4x36 – Episode 36
The gang tries to save Benson from an evil party company, who kidnaps and clones to sell their party-animal counterparts.
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4x37 – Episode 37
Mordecai plans to take Margaret out to a fancy restaurant so he can ask her to officially be his girlfriend.
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