Early on in Anna and the Apocalypse, the new Scottish Christmas zombie high school musical, the protagonist, Anna (Ella Hunt), hears a disgusting growling grunt from behind her. The audience braces itself, expecting that the promised zombie apocalypse has begun, but it turns out that it’s just a classmate of Anna’s, hacking her way through a winter asthma attack. Zombies are disgusting, it’s true, but humans are also disgusting. We’re basically fluid-filled staggering bags of meat. If you’re not looking carefully, and sometimes even if you are, it’s hard to tell the difference.

That’s the terror of zombie movies, and it’s also the subversive fun of them. As Peter (Ken Foree) famously muses in George Romero’s 1978 Dawn of the Dead while contemplating a group of shuffling corpses, “They’re just us, that’s all.” Zombies are parodies of humans, an exaggerated take on how we’re all ugly, clumsy, gross flesh puppets that don’t realize how close we are to the grave. The satire can be tragic and nightmarish, as it is in “serious” zombie movies, from Danny Boyle’s frantic 28 Days Later to low-key recent indies like Robin Aubert’s Les Affamés. But it can also be, as parodies often are, a joke. That’s why there are so many humorous zombie narratives, from Anna and the Apocalypse to Edgar Wright’s Shaun of the Dead to the ongoing Netflix sitcom Santa Clarita Diet. The point of the zombie genre is that humans are strange, repulsive, and ridiculous. When that insight isn’t specifically tuned to make people scream, it can be pretty funny.

That’s why more than half the time, even supposedly straight zombie films often seem like they’re going for laughs. George Romero’s influential 1968 film Night of the Living Dead is a somber affair, but it has the seeds of silliness staggering around in it. Romero took mass undead attack narratives like Richard Matheson’s 1954 novel I Am Legend, and made the monsters slower, dumber, and more corpse-like — which is to say, more human.

And while he’s more known for horror than slapstick, he frequently addressed the humor potential of humans running from clumsy pursuers or of zombies behaving like exaggerated humans. In Dawn of the Dead, zombies wander through a giant freestanding mall, tripping and falling into the fountain, or rotating haplessly on the escalators as a carousel-esque soundtrack plays. They’re driven by a half-understood longing for products that can no longer fulfill their bottomless hunger. “This was an important place in their lives,” Stephen (David Emge) muses as he watches the upright corpses lurch around the mall, just as, before the zombie apocalypse, upright corpses would lurch around the mall, driven by a need to consume. Later Romero films had different kinds of comedy: in 2005’s Land of the Dead, for example, the zombies are obsessed with fireworks, moaning and groaning as rockets explode in the air. They’re just as brainlessly distractible as they were when they were alive.

The film that shambled laughward with Dawn of the Dead’s example, though, was Shaun of the Dead, in which the central gag is precisely that humans are such sad, aimless, bland specimens that it’s barely noticeable when they start turning into sad, aimless zombies. Perhaps the film’s most famous scene (lovingly referenced in Anna and the Apocalypse) follows sleepy, staggering shop assistant Shaun (Simon Pegg) as he heads off to buy a paper in the morning. He’s so embedded in his dreary routine that he doesn’t notice the telltale signs of zombie infestation: people running in panic, bloody handprints on the glass case at the convenience store, and, of course, the undead making their stiff, vacant way down the street behind him.

Shaun even brushes off a zombie trying to get him by muttering that he doesn’t have any change. He sees a supernatural hunger and mistakes it for normal hunger. The final scene of the film shows Shaun’s useless friend Ed (Nick Frost), now a zombie chained in the basement, playing video games and communicating in monosyllabic grunts, much as he did when he was alive. Living or dead, the gag implies, our pastimes are meaningless either way.

Zombies by their (un)nature mock and simplify human aspirations, which lets them bring a satirical bite into more playful, optimistic genres as well as grim ones. Afterlife with Archie cheerfully drops bloody gobbets of doom on the carefree world of Riverdale. Marvel Zombies chuckles as Spider-Man eats Aunt May and a familiar set of costumed heroes murder everyone on the planet. Pride and Prejudice and Zombies, in both book and film forms, mocks the formal seriousness of Jane Austen, but it also implicitly reminds audiences that Darcy and Elizabeth are going to age, deteriorate, and molder forever. Zombies are a toothy refutation of happy endings, though not always of funny ones.

Anna and the Apocalypse pursues a similar bloody arc. Yearning Glee-ful early music numbers set up the expected teen longings: the girl who wants to escape her small town, the nice bullied boy racked by unrequited love, the queer kid whose parents don’t understand her. In the normal course of things, all of those characters would self-actualize and follow their bliss to one of those Hollywood endings they sing about in a choreographed lunchroom number. But instead… well, you know zombie movies. There’s a body count.

That body count in Anna and the Apocalypse is surprisingly ruthless, even by the standards of a zombie film. Zombie horror often tips over into humor, but by the same token, zombie humor can end up in some bleak places. One of the last scenes of the film shows former lovers, now zombies, pacing back and forth past each other, obliviously emotionless as their dead hands brush against each other.

The description “Scottish high school zombie Christmas musical” sounds like a lot of fun, but that moment of love swallowed by death isn’t fun or funny. It’s sad and ugly and a bit nauseating. People don’t care about each other when they’re in the ground, and when aren’t they in the ground? Zombie movies know that when we laugh at zombies, we’re laughing at ourselves. It sounds like the clotted hacking of the dead.