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Cats are fighting mice in an isometric screenshot from Mewgenics. The scene is set in a junkyard and there is a grid indicating where the cats and mice can move. Image: Edmund McMillen, Tyler Glaiel

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Mewgenics: The ‘sequel to Binding of Isaac’ is an infinitely expanding cat combat mashup

10 years after it was first announced, Mewgenics is finally rising from its litter box

Russ Frushtick is the director of special projects, and he has been covering the world of video games and technology for over 15 years. He co-founded Polygon in 2012.

Ten years ago, Edmund McMillen, the creator of The Binding of Isaac, announced Mewgenics, a cat-breeding life sim with Pokémon-style combat. At the time it was being developed as a Team Meat game, and was intended to be the follow-up to the wildly popular Super Meat Boy. Mewgenics was even shown at PAX 2013, but, according to McMillen, the project was canceled shortly after that showing.

In a tweet from earlier this year, McMillen wrote, “I never wanted to cancel Mewgenics way back when, but I had no control over it.” Speaking with Polygon, McMillen was unwilling to go into specifics regarding the split between him and fellow Super Meat Boy developer Tommy Refenes. But part of the arrangement handed the rights of Super Meat Boy over to Refenes and his team (who would go on to make Super Meat Boy Forever). McMillen, meanwhile, retained the rights to The Binding of Isaac, and would later secure the rights to make his own version of Mewgenics.

While it’s always been about breeding cats to some extent, this new version of Mewgenics has gone through several iterations. McMillen is now co-creating the game with Tyler Glaiel — another successful indie developer who last teamed with McMillen on The End Is Nigh — who started working on prototypes for what a new Mewgenics game could look like in 2018. At one point, the new Mewgenics was a Castle Crashers-style side-scrolling brawler (with cats).

Multiple cats fighting in an early prototype build of Mewgenics. Here we see the cats attacking a mouse with lightning and fire powers
Early prototype: When attempting to reboot Mewgenics, McMillen and Glaiel explored the RTS genre. This footage shows what the game might have looked like had it not switched to turn-based combat.
Image: Edmund McMillen, Tyler Glaiel

McMillen and Glaiel never figured out a way to make that work, though: “We knew that we wanted groups of cats,” says Glaiel, “and we couldn’t quite figure out how to work that into the brawler version of the game, because you can’t control four cats [by yourself]. As a brawler, it would have to be multiplayer. And I didn’t really want to deal with that. Not again.”

After the brawler idea, the team attempted to make a real-time strategy game (with cats), which Glaiel describes as “too chaotic,” requiring you to keep track of too many things at once. “It’s also not as tight as a brawler,” explains Glaiel, “so you can’t dodge abilities, because you’d have to click the cat and move it out of the way. […] It didn’t really work.”

In early 2020, after several iterations, the team landed on the idea of a turn-based tactical strategy game (with cats), and that’s what stuck. But this isn’t just a Fire Emblem knockoff. It’s one of the most ambitious and bizarre strategy games I’ve ever played.

What has Mewgenics become?

Cats fighting other cats in a screenshot for Mewgenics. Here we see two armies of cats in a junkyard. There’s fire spreading and one of the cats has a paw above its head to indicate that it’s about to attack Image: Edmund McMillen, Tyler Glaiel

It’s easiest to think of Mewgenics in two parts. The first part is the tactical combat game, which should feel familiar to anyone who has played Dungeons & Dragons or Into the Breach. You start with a squad of four cats with traditional role-playing classes like hunter, mage, and healer (among others). Each of the cats has different stats and abilities, so one might be great at backstabs with a high movement speed, while another hangs back and casts spells. You’re managing mana and equipping gear that will give each cat an edge in battle. At the end of each battle you’ll level up one cat and select a new ability that pairs with their current loadout.

On the surface, this all feels pretty typical for the genre, but McMillen and Glaiel are creating something far more complex and variable.

A simple example: You can send one cat to hide in grass and it’ll increase their dodge chance. But grass is flammable, so if you’re attacked with a fire spell, that cat will be set ablaze. Fire can also spread so long as there’s debris nearby, which means the whole map might quickly go up if you’re not careful.

the equipment screen of a cat in mewgenics. the cat is named Punchy and it is wearing a cruddy wool hat and has a knife equipped. the inventory screen is also visible, showing different hats and accessories, like a paper boat hat Image: Edmund McMillen,Tyler Glaiel

Weather can also change that same scenario entirely. If it snows, all of the grass on the map will turn into damage-dealing shards of ice, which can threaten the safety of your cats or create defensive barriers to keep enemies out.

During my play session, cause and effect kept rearing its head. One of my cats had the ability to poop on command. The poop did no damage, but did create a barrier to block enemy ranged attacks — until I realized that the poop would go flying if hit by an enemy, directly into the cat who just dropped it. These sort of ricochet scenarios constantly play out in Mewgenics, requiring the entire battlefield to be surveyed before a move is made.

The potential elements are so varied that Glaiel had to create an AI program that ran through thousands of battles at lighting speed, just to make sure the game didn’t generate unwinnable scenarios.

I had a particularly devastating moment where I fought a giant mouse with sunglasses (a nod to a classic Super Mario Bros. 2 boss) who kept tossing bombs that would explode in long lines across the length of the map. While I was able to approach and disarm the bombs thrown, the defused bombs remained. I should have probably anticipated that using an area-affecting fire spell near one wasn’t the best idea and yet, there I went, fully devastating half of my team in one flammable mistake.

So what happens if a cat dies? A cat losing all of its hit points is not the end of that cat. In Mewgenics, these cats will be downed for the battle and, so long as they don’t get hit three more times in that same battle, they’ll survive for another day… but with consequences.

One of my favorite cats, a combat healer named Mystic, took one of those bombs to the face and ended up with a concussion, impacting his mobility and ranged attack ability. And he got off easy. Glaiel mentioned an injured cat might literally “lose its balls,” which leads us to the second major portion of Mewgenics.

The cat breeding part of Mewgenics

Completing a “run” of Mewgenics will see your surviving squad of cats make it back to your house. They’ll bring with them some of the equipment and abilities they picked up along the way, which can be equipped to other cats on future runs. Two cats can also be stuck in a room together and, if things click, you’ll end up with a baby cat with components pulled from the two parents. The kitten might have the father’s ears, the mother’s coloring, and a mutation passed down from the father, like fangs that grant life steal on attacks. On very rare instances, you might pass down an active or passive ability, pulled from the classes of the parents, letting you multi-class cats if the stars align.

The team’s goal is that you’ll obsess over crafting the perfect genetic specimen, an unstoppable killing machine that they liken to breeding in Pokémon games.

But there are two key differences between breeding in Mewgenics and breeding in Pokémon.

For one thing, you can’t breed the same super cat with every other cat in the house, because you’ll end up with an inbreeding situation. “If you inbreed your cats too much they will turn out fucked up!” says Glaiel. “I have a scientifically accurate inbreeding sim in there... I read a bunch of academic papers about dog breeding databases to figure that out.”

Leveling up a cat in Mewgenics. A cat named Zora is leveling up at the end of a battle and is able to select between four abilities: Rally Charge, Witch Hunt, Wish, and Feather Feet. Each of the abilities has different statistical advantages depending on the cat’s class. Image: Edmund McMillen, Tyler Glaiel

The other important difference: Sorry if this comes as news to you, but cats get old and die. After enough time has passed, eventually your cat will become too old to go on adventures. They’ll hang out in your house, and you might be able to use them for breeding, but their fighting days will be over. Eventually the cat will die of old age, with only its feline lineage speaking of its contribution to your world.

So yeah, you just have to accept that your favorite cat in the game, the cat you started with and grew super attached to, will die. It might even die because of a dumb mistake you made in the middle of a random fight against some puny mobs.

“Part of what the game is is accepting the fact of permanence,” says McMillen. “That sometimes bad shit does happen … [but] the things that happen create this element of a living world that, once the rules are established, and you know that bad stuff will happen, you accept that. You accept the rules of the game.”

In most cases, losing a cat won’t be a big deal, says McMillen, because you’ll have a house full of strays. “But maybe there is this one cat that you really love. And maybe this cat’s getting old. And you’ve got to do something with this cat and you know that the permanence of this cat dying is a real thing. And it’s gone forever. That’s what we’re trying to make.”

The idea of permanence extends to items, too. There will be common and rare items that you’ll pick up to grant your cats stat and ability boosts, and the same item can show up more than once across your adventures. But there will also be legendary items. McMillen mentioned an item like the Holy Grail that can show up just once per save file... a save file that might have 300 hours of progress. It’ll be an ultra-powerful item, one that gives you a huge edge in combat, but if the cat that’s holding it dies, it’s gone forever. You’ll never see it again. “There’s only one Holy Grail, after all,” says McMillen. Generally people still store these legendary items in their house and, if the time comes when they really need that extra advantage, they can pull them out, factoring in all of the risks associated with them.

How does it all end?

McMillen and Glaiel are intentionally vague about what it’ll take to “win” Mewgenics, but they’ve shared some vague outlines with me. After a certain number of adventures and virtual days have passed, your house will be invaded by some sort of boss making demands. You can send them on their way by giving them valuable items or even the souls of cats (presumably ones you’re not super attached to). But you can also choose to fight back, and the only cats that will be able to defend the house are the old, veteran cats that can no longer go on adventures. These boss fights will be major act breaks in Mewgenics, eventually leading to an epic final fight.

Beyond that, though, the team isn’t super open about specifics of the endgame. All they’ll say is that they’re designing Mewgenics to be incredibly modular. They want to make a game that could extend on for years (or even decades) through content updates after the launch (which the team estimates is about 18 to 24 months away). It follows in the footsteps of The Binding of Isaac in that way, which first launched in 2011 and continues to get updates and expansions to this day.

“I really see Mewgenics as the sequel to The Binding of Isaac,” says McMillen. “In terms of how you take something like Isaac and make it into a bigger and better thing, expand on it indefinitely. Mewgenics is that.”