The show’s foreground is jaunty, but its background hints at a ruined world. Illustration by Pendleton Ward

The animated series “Adventure Time,” now entering its sixth season on Cartoon Network, is the kind of cult phenomenon that’s hard to describe without sounding slightly nuts. It’s a post-apocalyptic allegory full of helpful dating tips for teen-agers, or like World of Warcraft as recapped by Carl Jung. It can be enjoyed, at varying levels, by third graders, art historians, and cosplay fans. It’s also the type of show that’s easy to write off as “stoner humor,” which may be why it took me a while to drop the snotty attitude, to open up and admit the truth: “Adventure Time” is one of the most philosophically risky and, often, emotionally affecting shows on TV. It’s beautiful and funny and stupid and smart, in about equal parts, as well as willing to explore uneasy existential questions, like what it means to go on when the story you’re in has ended.

If that sounds pretentious, there’s definitely a simpler way to watch the show: as a cartoon about a hero who fights villains, with fun violence, the occasional fart joke, and a slight edge of Bushwick cool-kid hipness. When it began, in 2010, “Adventure Time” stuck more closely to a familiar formula, in which a good-hearted human boy named Finn and his dog, Jake, a gruff-voiced wingman type, were brought up as siblings. For no clear reason, Jake can stretch to any length and alter his body, extending his front leg to make a bridge, blowing up to mountain size, or turning himself into an armchair. Finn and Jake’s friendship has a classic nerd-and-his-id dynamic, a bit like Calvin and Hobbes, in which a wild animal encourages his friend to go on adventures and cheers him on with girls (among them, the scientist-princess Princess Bubblegum). Online summaries make it sound like a wacky romp: “Finn and Jake are assigned to watch three magical beans; two of the beans are innocent, but one of them is evil. . . . Eventually, one of the stalks produces evil piglets, and Finn and Jake defeat the beings using a novel method that involves ice cream.”

But, as the series progresses, an eerie backstory emerges. The candy-tinted world we’re seeing has a terrible history: while Finn is surrounded by magical beings, virtually every other human appears to have been killed or transformed, during something called the Mushroom War, which included, according to one unnerving voice-over, “frightful bombs poised to bathe the land in mutogenic horror.” As a baby, Finn was left on a leaf, crying and alone. The other major characters have their own disturbing origin stories. The sociopathic Ice King, who keeps trying to kidnap princesses and marry them, used to be a gentle antiquarian named Simon, a personality he lost while fighting mutants after the war, when a magic crown drove him crazy. The punk-goth vampire Marceline has a mobster-like dad, who rules an underworld called the Nightosphere, but she, too, wandered alone in a postwar landscape as a child. At the age of seven, she was rescued by Simon, but once he became the cackling Ice King he forgot he ever knew her.

As with “The Simpsons,” the ensemble is enormous, allowing episodes to go off on odd detours and tell the stories of minor figures, such as the woozy Southern elephant named Tree Trunks; or the freaky Lemongrabs, lemon-headed maniacs who spend most of their time howling in frustration; or the friendly Korean-accented computer named B.M.O. In later seasons, these threads cohere into a broader cosmology; it includes an alternate time line, in which the bombs haven’t yet exploded. Finn is the only character who ages in a normal fashion: he began as a twelve-year-old, but this season he’s sixteen, and his relationships, and the show itself, have become deeper and (codedly) more sexual. There are moments when Finn’s story feels suspiciously like a compensatory fantasy, invented to disguise a trauma that can’t be faced head on—as if it were the “Mulholland Drive” of children’s television.

In interviews, Pendleton Ward, the show’s creator, has laid out some of his influences, including the role-playing game that was the leisure-time activity of my own nerdy teens, the dice-and-paper-based Dungeons & Dragons. Many of the villains, like the skeletal Lich, have visual origins in the old D. & D. “Monster Manual”; the plots include classic quest-style puzzles—find the gems, open the portal! In the hand-drawn pastel backgrounds and the characters’ bubble eyes, there are hints of Japanese animation—Ward has cited the Hayao Miyazaki movie “My Neighbor Totoro” as an inspiration—but the aesthetic feels equally informed by the urbane squiggles of Felix the Cat, the jolting aggression of early Mickey Mouse cartoons, and the bratty buddy energy of series like “Beavis and Butt-head” and “Ren & Stimpy.”

No matter how jaunty the foreground, though, with its penguin named Gunter and its goofy shouts of “Youth culture forever!,” the background hints at a ruined world: missiles poke out, and abandoned technology is everywhere. Bodies morph grotesquely—when Marceline’s father gets angry, he transforms from a businessman into a demon with an ass face, a vertical orifice for a mouth and a tentacle beard worthy of H. P. Lovecraft—and these ugly visions suggest both the horrors of adolescent bodies and postwar mutations.

Such darkness might easily turn cynical, and yet “Adventure Time” has a gentle heart. Even characters like the ridiculous Lumpy Space Princess, a brash Valley Girl who is voiced by Ward, have layers, and moments of pathos. Take “Bad Timing,” an episode from the fifth season, which opens with an odd image: a large circle in the center of the screen, like a porthole. Peering into it, we can see Princess Bubblegum demonstrating her newest invention, a time machine called a “phasical sphere.” When Bubblegum places this sphere over the circle, she seems to break the fourth wall, turning the TV into a lens. Then she shouts, “Now check it out! It’s logging time.”

What follows is a tragicomic fable in which Lumpy Space Princess, while on the rebound from a breakup, gets into her first real romance and then, swamped by insecurity, wrecks it. As we peer at various scenes through the circle—Lumpy shrieking “You skunk! You pretty skunk!” at Bubblegum, then, later, tossing a root-beer Molotov cocktail at an imagined rival’s castle—there’s a strange flickering. In the corners of the screen are tiny creatures: two triangles fishing in a creek; a diamond doing curls with a dumbbell. These miniature dramas don’t affect the story, yet we’re forced to glance at them, like footnotes, or a silent chorus; they suggest a larger universe of loneliness and connection. In the final scene, after Lumpy has accidentally dissolved her boyfriend in the phasical sphere, she begs Bubblegum, “If he’s gone, can you send me back, to before I met him? So I won’t have to remember this heartache?” A tiny shape hovers nearby, seeming sympathetic. Bubblegum grants the request, and blue rivers spill from Lumpy’s eyes. Just before the credits, the borders beyond the circle fade to black, and the small chorus disappears.

Of course, this description may leave you cold—like any trip, “Adventure Time” has a definite “you had to be there” quality. It’s a dreamlike experience, and a druglike experience, and we all know how much people enjoy hearing other people describe those. (Luckily, each episode is eleven minutes long, so this is not the same as when your friends were nagging you to watch “The Wire.” Five late-season episodes should suffice.) But that’s part of the show’s most freeing quality: childlike, nonlinear, poetic, and just outside all the categories that the world considers serious, it’s television that you can respond to fully, without needing to make a case for why. Here. Have a taste. ♦