The episode opens with Marceline, Finn, Jake, and the Ice King playing basketball at Marceline's house. Finn and Jake question Marceline on why she invited the Ice King, she then explains to them about her long history with the Ice King. The two request her to tell them about their past, as the Ice King comes back from getting water, agreeing.
She begins telling a story about the two of them (exactly) 996 years ago. It starts with 47-year-old Simon and 7-year-old 'Marcy' (Marceline) wandering through the empty ruins of human civilization, destroyed in the Great Mushroom War, searching for food. At the edge of the forest, they find a mailbox but there is only a dead rat inside. Simon tries to keep Marcy's spirits up by joking around, remarking that the rat will be a cute new traveling partner. He discovers a VHS tape nearby on the ground, and suggests they "watch" it - without a VHS player or electricity, he literally stares at the tape, then jokes that it was a boring movie and says the book is better.
They then hear a rustle from the woods, Simon then puts his crown on to protect him and Marcy, even though Marcy is clearly afraid of what the crown does to him. Simon reassures her and says he has to to keep her safe. Once Simon puts the crown on his head, its magic makes Simon forget himself. Marcy begs Simon to take off the crown. It turns out that the rustle is caused by a deer in the bush, but Simon has no control of himself as he blasts the deer with ice anyway. He mistakenly calls Marcy "Gunter" and tells her to play in the snow as he generates snowflakes to cover her. When Simon is using his freezing power on the deer Marcy throws a snowball to knock Simon's crown off. It takes a little while for the effects from the crown to wear off, and this worries Marcy. She then makes Simon promise never to put it on again. Marcy does not know what the animal is, and Simon explains that it is a deer. Marcy wonders that if the deer was alive in the woods, maybe other things have survived the war out there as well. Her comment underscores just how empty the wasteland is, with the pair not even knowing if other beings are alive despite months of wandering in the ruins. Simon pauses and ponders the question, but honestly does not know.
At the end of the day, Simon sets up camp for the two of them. The deer he froze in ice has solved their food problem for the moment, and Simon cooks it over their campfire. He tries to make her laugh by acting out an episode of Cheers for her through a broken television. Marcy then taps an old soda can and says, "mute." Simon mouths words as if he was actually muted. Marceline has a good laugh, but then begins to cough, making Simon realize she has a low fever. He tells her to sleep to preserve her strength.
Simon decides that Marcy needs chicken soup, and remembers that he saw a "Soupery" shop in a town they recently passed. Not wanting Marcy to walk all that way, he finds an abandoned motorcycle, but loses control when he tries to ride it, and they bail out before it crashes into a wall and explodes. Still not wanting Marcy to walk all the way, Simon has Marcy ride on a wheeled chair that he pushes all the way into town. He scat sings along the way to cheer up Marcy. Since the door to the soup shop was locked, Simon breaks a window to get into the soup shop, though he tells Marcy that vandalism is wrong. The shelves of the soup shop are all empty (except for some soup-themed novelty gift cards). Just then, Marcy looks in the security mirror in the end of the aisle, and notices something, shambling along in the next aisle. Simon peers over the shelves and is shocked to see some sort of weird mutant slime-creature. Alerted to their presence, it advances towards them, but Simon knocks the creature out by smashing its face in with his crown, then they flee.
Later, Simon and Marcy quest deeper into the abandoned city, by walking through a large concrete storm-drain. Marcy seems worse, but Simon notices a food truck abandoned on a broken bridge that passes over the storm-drain up ahead. They climb up, only to see it is a clam truck called, "The Clambulance," a frustrated Simon asks Marcy to cover her ears as he "curses" and kicks the truck. Suddenly, slime creatures emerge from inside. Simon then pushes the truck off of the bridge with the slime creatures on it, but the truck's siren starts going off after it crashes into the ground. This attracts a horde of the slime creatures, who chase Simon and Marcy to a dead end in an alleyway. Simon is forced to put his crown on, and tries to stay normal by singing Gary Portnoy's "Where Everybody Knows Your Name" (the theme from "Cheers") repeatedly, and after knocking all of the creatures out, takes his crown off. However, this time, most effects of the crown do not go away (his beard is the only thing that shrinks back to its normal length). Chicken soup and a can opener then fall from a pink substance clinging to the side of a building. Upon closer inspection, it appears to have a face, which smiles at Simon. He then uses the soup to cure Marcy. Marcy tells Simon she loves him and hugs him. Simon then says "I love you... Gunter."
The episode then goes back to the present. Marceline finishes the story, by saying that she gets better, and she and Simon live happily ever after. He then tells her to keep telling "these chumps" stories while he scores more baskets, proving he has no recollection of the events himself. The episode ends with Marceline, Finn and Jake smiling at the Ice King being happy.
- This episode was nominated for the 2013 Emmy Award for the category "Outstanding Short-Animated Program."
- As of May 2016, "Simon & Marcy" is the highest user-rated episode of Adventure Time on IMDb.com, with a rating of 9.4; the average Adventure Time episode rating is 8.0 by comparison.
- Marceline is seven years old and Simon is forty-seven in the flashback in this episode meaning that their current ages would be 1003 and 1043 respectively. This would also mean that Marceline was exactly 1000 and Ice King was exactly 1040 at the start of the series.
- The dried river they are walking through shortly before coming across the "Clambulance" greatly resembles the Los Angeles River. This implies that the city is indeed Los Angeles.
- Simon calls Marceline "Gunter" with the crown on. Later, toward the end of the flashback, he does this again with the crown off. This is an indication that Simon has fallen deeper into his transformation into the Ice King.
- The Chinese characters 水 ("water") and 人生 ("human life") appear on a bottle inside the first car Marcy hides in. This could also be interpreted as Japanese kanji. In addition to this, in the upper-right-hand corner of the broken television, the Japanese characters such as テレビ ("television") can be found.
- The overcast skies seen throughout the episode are likely a result of Nuclear winter.
- Simon has drumsticks in his backpack, hinting that he already knew how to play the drums.
- By the Clambulance when Simon looks down to find a radiated creature below the truck, the registration plate of the truck says "CLAM 1" "The Wut-Wut state."
- A photo of a mushroom cloud can be seen in the mailbox at the beginning of the episode.
- The snail in this episode is seen during the flashback. This could mean the one seen is a surviving ancestor to the snail seen in every other episode. This however could just be an oversight by the producers.
- In the back of the car that Simon puts Marceline in at the beginning of the episode, there appears to be a magazine with what seems to be a picture of 3 planes, flying.
- The numbers 666 appear on the facade of the building near the pickup truck. And in front of this building there is a fountain in ruins that resembles the Universal Studios Hollywood Globe.
- Simon and Marceline are sitting on a rocket without a head when they are eating by the fire.
- When Simon jumps out from behind the shelf of greeting cards, he knocks a few over that read: "DON'T 'LADEL' ME," "HAVE A 'SOUP'-ERIOR HOLIDAY," "SEE YOU SPOON," and "BOULLIONS AND BOULLIONS."
- The transitions "ON THIS DAY 996 YEARS AGO" and "996 YEARS LATER" are very similar to those in "Return to the Nightosphere" when Finn and Jake are waiting several days on the line.
- For some unknown reason, Simon has a Scientific Parasite in his coat pocket throughout the whole episode.
- According to Supervising Director Adam Muto, Mother Gum and its narrative role in this episode was partially inspired by the "Fungus King" from 1993 live-action film Super Mario Bros.: The Movie.
- The transitions "ON THIS DAY 996 YEARS AGO" and "996 YEARS LATER" are very similar to the ones used in the zombie film, 28 Days Later and its sequel, 28 Weeks Later.
- Ice King sings "Where Everybody Knows Your Name," the theme song for the television show "Cheers." He likewise calls out for Norm, a character on the show, and re-enacts an episode.
- After singing the Cheers theme song the first time, Simon said "Filmed before a live studio audience," a quote commonly used by announcers of live television shows, most notably in Cheers itself.
- The red car that Simon puts young Marceline in is a 1971 Nissan Skyline GT-R (KPGC10) "Hakosuka." A brief shot shows the "R" badge on the side and an even briefer shot shows the "R" badge from the front. It is interesting to note that the car is left-hand drive, whereas the real car is actually right hand drive.
- After Simon puts Marceline in the red car, a green CD case with The Cure "Boys Don't Cry" photo on the front cover can be seen in the middle compartment. The same picture previously appeared in the episode Marceline's Closet. This time it's even supported by the word "CURE" on the spine of the CD case.
- The motorbike which Simon and Marceline rode is a 1977 Kawasaki KZ650.
- The Clambulance resembles a 1983 Bedford CF Ambulance.
- Simon's reaction upon discovering that they had discovered a Clam van instead of a Soup van was a reference to Woody Harrelson (formerly of Cheers)'s reaction to finding Sno-Balls instead of Twinkies in the film Zombieland.
- This episode, as noted by critics, bears similarities to the 2006 dystopian novel and 2009 film "The Road." The novel takes place in a post-apocalyptic America after an unstated event had destroyed civilization and majority of the human race, with many people succumbing to cannibalism. The novel focuses on two main characters, an unnamed father and his son, as they try to stay alive and keep from succumbing to the horrors they encounter. Simon and Marceline take the roles of the two main characters, with Simon taking the role of the father and Marceline taking the role of the son. The mutants they encounter arguably take on the role of the cannibals encountered by the pair and the Ice Crown could be a reference to the revolver the pair carry.
- The alarm from the Clambulance alerting the horde of Goo Monsters may be a reference to several stereotypical survival horror games, in which the sound of triggered car alarms can alert hordes of zombies of the whereabouts of the survivors.
- When reaching for a weapon in the pickup truck to fend off the Goo Monsters, Simon grabs a pillow when there was a crowbar right beside it. This could be a reference to censorship of various degrees of violence in cartoons in which weapons are sometimes replaced with comical objects as censors see fit. The presence of the crowbar could have also been a possible reference to the Half-Life series.
- The scene where Simon and Marceline approach the Clambulance on the edge of the bridge resembles a scene in the episode "Consumed" of season five of "The Walking Dead", which aired an year later:
- Both scenes feature a male and a female scavenging in the Post-Apocalyptic world.
- They both scavenge a vehicle on a bridge.
- Both vehicles eventually fall from the bridge, which alerts several zombies in the area.
- This episode title is similar to that of "Lady & Peebles." The episodes are also pairing in the fact that they feature two supporting characters adventuring without Finn and Jake.
- Marceline catches the cold right after Simon sprinkles her with his snow. This could be a reference to the episode "What Have You Done?" where the Ice King's beard flakes accidentally make the citizens of the candy kingdom sick. This could mean Simon was unintentionally responsible for Marceline becoming ill.
- Marceline now hanging out with the Ice King is caused by the events in "I Remember You." Their hundreds-of-years relationship was begun to be revealed in this episode.
- This episode was first aired as a two-part special with the previous episode, "I Remember You," similar to "Bad Little Boy" originally airing with "Fionna and Cake."
- Marcy said to Simon "Your songs are so good" is a reference when Ice King said "Marceline's songs are so good" as heard in "I Remember You" perhaps where Simon got it from.
- Simon says "Ice winds, BLOW!" while freezing the deer, which is what he said in "Hitman" when he blew away the smoke Scorcher made to suffocate Finn and Jake.
- The pink substance on the wall has been revealed to the Mother Gum, as seen in Adventure Time: Explore the Dungeon Because I DON’T KNOW! and "Bonnie and Neddy."
- The mutant creatures trying to harm Simon and Marcy return in the later episode "James."
- Before Simon was going to hit the monster with his crown, it was facing forwards, but when he lifts it above his head to hit the monster, he is holding it facing the other way. The crown changes back to facing forwards after hitting it.
- The drumstick in Simon's backpack with tape on it switches sides when Simon is talking to Marceline about wanting to carry her.
- When Marceline is talking to Finn and says "He's very dear to my heart" her bite mark seems to be farther towards the back of her neck than usual.
- When Marcy says "Simon you promised," Simon's crown is missing a gem.
- After Simon had knocked down the creature inside the soupery, Marceline's sock was on her right foot, contrary to the rest of the episode, where it is on her left.
- When Simon and Marceline were on their way to the bridge where the clambulance was, the remote she stepped on and the big crack on the wall that was seen when Simon carried her were not there.
- The Japanese characters were not seen in the upper-right-hand corner of the broken television until Simon realized Marceline was sick.
- The last time Simon puts his crown on, his coat is black, but when he takes it off, it is blue, and it stays that way until the end of the episode, although the coat color could have been affected by the cold.
- This is the final episode that Rebecca Sugar worked on before she left to create Steven Universe.
- During the first airing of this episode, the title card (along with the opening credits) was never shown.
- This episode was leaked before its air date.
- The mutagenic effects of the Mushroom Bomb/Mushroom War can be seen in the appearance of creatures oozing green slime from open pores throughout their bodies.
- Marceline now seems to feel more comfortable around Ice King since "I Remember You" (where she tells Ice King not to come around her) as this time she invited him to come over and also told Finn and Jake about their past.
- The events of this episode's flashback seem to take place before the first memory in "Memory of a Memory" and after the flashback in "I Remember You." In the flashback of "I Remember You," Marceline is a toddler (as revealed by her model sheet), while in this episode's flashback, she is seven years old; more evidence of this is that her clothes in this episode seem to be slightly more ruined and tattered. She still appears smaller than herself in the first memory in "Memory of a Memory," and her clothes appear slightly less tattered.
- The pink substance is the Mother Gum, showing the origins of the candy people. More specifically, it's the ancestor of Princess Bubblegum, and she is the one who created the whole Candy Kingdom and its inhabitants in the early stages as seen in "The Vault."
996 years ago: