Kyle is playing with fire when he gets in the middle of Cartman and Heidi’s relationship.
The episode starts with Heidi sitting in bed when Cartman calls. Cartman is sobbing on the phone begging her not to break up with him. He says his blood sugar is the reason why he acts the way he does. Liane comes in and Cartman cusses her out. Heidi tells Cartman he needs to become a vegan. Cartman agrees to do so.
At South Park Elementary, the boys are eating lunch. Cartman has a vegan dish in front of him, he complains about Heidi controlling him. Kyle steps in and tells Cartman he is sick of him calling Heidi names. Heidi comes over and offers for him to eat with her. After they leave, Kyle again bashes Cartman.
The school is letting out, Kyle asks to talk with Heidi. Kyle asks about her relationship with Cartman, Heidi says the girls bash her for going out with him. Heidi becomes angered and starts yelling at Kyle, she storms off, leaving Kyle stunned.
Cartman knocks on Heidi's door, with a bag in his hand. He comes in with "Beyond KFC" which is actually normal KFC. Cartman says it's great to be vegan. He gives off an evil grin.
At South Park Elementary, Kyle talks with the boys, saying he found out why Heidi won't admit Cartman is a horrible boyfriend. Cartman comes over, saying she's gained some pounds. Heidi comes over, complaining about not feeling well as Cartman looks back at her stomach and at the guys. Heidi says she wants to come home, she and Cartman go walk over to the nurse's office as Kyle says they must help Heidi.
At the White House, President Garrison is insulting a world leader on the phone as his advisers come in announcing that a worker with ties to Russia was found "raped and executed". Garrison says he "fucked him to death", the Member Berries start membering' as Garrison tells his advisors to put a positive spin on the issue. Paul Ryan says it's time for them to "put their foot down", Garrison pushes a button which locks the doors.
The girls are playing volleyball when Kyle walks in. He holds his hands up as he walks to the center court. He gives a speech about why all boys aren't bad. Throughout his speech, he makes various mistakes. Wendy finally asks him what he wants, Kyle responds by telling them they shouldn't criticize Heidi for going out with Cartman. The girls suggest Kyle likes Heidi, Kyle responds back defensively.
Kyle is sitting alone at school when Butters comes over to him. He says it's their moral obligation. Kyle says they aren't getting involved and he walks away.
Kyle goes over to Cartman and wants to talk to him. He is setting up a drum on his chest, as Heidi walks through, he starts making fun of her.
A breaking news icon shows up on the screen, a reporter talks about President Garrison calling the people Saudi Arabia "Dirty sand Nigers". Paul Ryan, who has a black eye with cum on it says the president is doing "great". Paul Ryan says he has "doorknob cum" on his eye.
At South Park Elementary, Heidi is sitting in the gym. Kyle is standing in the doorway looking at her. He comes over to her and sits by her. Heidi says he's waiting for Cartman to change. Kyle tells her Cartman isn't going to change because he sees himself as the victim all the time. Heidi says she thinks she's a bad person for following her heart.
A montage starts playing as Heidi is looking through her memory book with pictures of her and Cartman. She is at her locker when she looks over at Kyle, who is also at his locker down the hall. Cartman is yelling at Heidi on a bench, she goes up to walk away as Cartman pleading for her not to leave. Kyle is in the hallway, opening a letter which makes him smile. Garrison is talking with a foreign leader as people watching in the Crunchy's Micro Brew look on in disbelief. Heidi is in a restaurant with her parents as Cartman looks on in the window begging her. Heidi and Kyle are shown sitting together on the swings.
Cartman knocks on Token's door. Token answers, Cartman is sobbing hysterically. He eventually spits out that Heidi broke up with him. He begs to stay with Token. Saying he wants to go "disrespect the flag and flip over cars". Token slams the door shut. Cartman repeatedly rings the doorbell and Token answers it once again. His mom now with him. Token's mom invites Cartman inside.
At the White House, Garrison's advisors are meeting in a room discussing his new approval ratings. Garrison walks in on them and asks what's going on. He looks at the folder with the approval ratings and says "everything is going to be fine".
Token, Cartman and his family are eating dinner. Cartman asks what time they are going to go out and "disrespect the flag". His parents grow agitated and Cartman begs them saying he doesn't know how to feel. Token's dad says he saw Heidi holding hands with Kyle at the park. Cartman becomes filled with anger. A montage shows with Kyle marching on the screen, more and more Kyle's show up. A heart is shown exploding with multiple Kyle's walking through it. People are shown dancing in circles. Cartman lights a flag on fire, a very large Kyle surrounds Cartman and Heidi, who are holding each other. The people dancing are shown again, the ground crumbles beneath Heidi and Cartman as Kyle explodes out of the ground. It goes back to Cartman eating dinner.
At South Park Elementary, Kyle is walking through the hallway when Cartman screams his name. Kyle stops walking as Cartman approaches them, they get into a circle as others start to gather. Cartman pushes Kyle against the locker as he says it's time for him to fight back. Cartman goes to punch Kyle as Kyle responds with a single punch, which sends Cartman to the ground.
At the Buca De Faggoncini, the girls are meeting at a table. Cheering Heidi for "coming to her senses". They start bashing her for dating Cartman. She starts to get depressed saying she "it was just what I believed in".
Cartman is sitting alone at the park when Heidi comes over to him. She said she wants to make sure she's okay. Cartman says none of this is her fault and that it's time he tells her stuff.
At the Broflovski Residence, Kyle is putting hair gel in his hair when the doorbell rings. He goes downstairs and opens the door, Heidi is at the door as Kyle asks if she's ready to go. They both hold each other's hands as she tells Kyle she was being manipulated by him. He tries to explain himself but she stops him, explaining that she is Irish and that makes her moody and that Kyle can be sneaky sometimes. Heidi goes to walk outside after saying that it's hard to live life with Kyle's people getting in the way. Kyle stands in shock, saying that she just called him a "Dirty Jew". She walks over to Cartman, who gives Kyle an evil look. They walk off as Kyle watches in disbelief that Cartman got her back.
At the White House, a "Happy Anniversary" cake is carried by Paul Ryan. They bring it up to Garrison, who blows out the candles and everybody in the room starts cheering him. Garrison stands up with a strap-on penis, others in the room start nervously applauding.
AV Club gave "Doubling Down" a "A-" rating saying: "And that’s where Parker and Stone leave us, with another ending that never settles on a definitive answer. But where more recent South Park conclusions of this ilk have resulted from the show’s creators refusing to anchor themselves to a specific viewpoint (sometimes frustratingly so), “Doubling Down” simply recognizes that this past election was complicated. It recognizes that Trump voters, like anyone else, are complicated. Should their opposers embrace them or should they never bother reaching out in the first place? In Parker and Stone’s eyes, that’s something that can’t be answered by a mere cartoon.."
IGN gave "Doubling Down" a "6.3" rating saying: "Had this South Park episode focused exclusively on Cartman's rapidly deteriorating romance with Heidi, it could have been another winner for Season 21. The best moments this week were those focused on Cartman's dark schemes and the return of his feud with Kyle. Unfortunately, the emphasis on pointless Oval Office humor weighed down this episode pretty severely. It's just more evidence that South park needs to keep easing up on the political humor for a while."