20 Second Multiverse! ~Reincarnation With a Heart That Doesn't Beat~
廿秒間の多元宇宙（マルチバース）！ 〜鼓動しない心臓の転生〜
(Hatabyōkan no Multiverse! ~Kodō-shinai Shinzō no Tensei~)
As the title implies, the main character's heart does not beat. So he falls unconscious a bit under 20 seconds after being summoned and dies around three and a half minutes later. He's summoned to a new "other world" each time this happens, though, and so he's the only one that can save the world from the multiversal threat of MISTER POWERSCALING...
...And assemble a harem of catgirls along the way, of course. (They're all catgirls because each universe has a designated catgirl protector. But it's totally fine for the MC to take them from the universes they're supposed to be protecting. He's more important.) They can travel with him just by holding his hand when he dies, but obviously sometimes they don't make it in time, which is super duper sad and the first time this happens there's a whole five chapter arc about the main character being sad about it. Whenever it happens after that, he's just sad for one chapter; we don't got time for that.
Oh, but the first catgirl is, like, extra magical and can just travel between universes on her own, 'cuz we've obviously gotta have a main love interest to put on the volume covers someone that can be his totally platonic equal and the one person who can truly understand his suffering! And she's the guardian catgirl of the whole multiverse or whatever.
Why doesn't she just go bring the left behind catgirls to the new universe herself...? I guess because she can only go to each universe once, yeah. Except for when the main character goes back to the very first universe for the climax of the story, because then, uh, that one's special because of MISTER POWERSCALING's multidimensional magic or...
Oh, wait, no! If she goes back, she'll lose her connection to the MC and not be able to pinpoint his location in the multiverse anymore. Yeah, yeah. See, this is good, because then we can have a super important catgirl (she has the power to seal MISTER POWERSCALING or whatever and they spent half the story looking for her) get left behind and the main character gets to have a cool moment where he tells the main cat girl to go back for her and she says okay with tears in her eyes. It's a two-page full-color spread.
Then, we get a short arc entirely from her perspective, where she's searching the endless universe to find the world that the MC is in, gathering up all the lost catgirls as she comes across them. The whole while, there's a countdown in upper left corner, counting down (represented with weird esoteric symbols, though you can tell they're going down, because the number gets shorter) until MISTER POWERSCALING destroys all of reality. Next to that, there's a counter telling you how many universes there are left to explore. (The number starts out so long it falls off the page, into the one next to it, and then off that page, too.)
In later sections, the esoteric numbers eventually turn into years and the "universes left" number gets short enough that you can see the end of it. If you do the math here, you find out that she's spending a bit less than 20 seconds in each universe. Eventually, you get to "UNIVERSES LEFT: 1." and "TIME UNTIL THE GREAT UNDOING: 20 seconds." She closes her eyes, then travels to the final universe. (Ignore the plothole of the fact that the MC's also constantly moving and might be in a universe she's already explored at any given moment. They all get destroyed by MISTER POWERSCALING's superweapon when she leaves, so he can't be in them.)
Then, we cut back to the MC and see what he's been doing all this time. Basically a lot of dying.
Okay.
He has a monologue as all that happens and different worlds flash around him, but then he opens his eyes to the final universe, which it turns out was the first universe all along. He then sees the main catgirl (with the rest of the harem in a stampede behind her) running up to him in his final seconds before everything is destroyed in a flash of white light.
The two of them (plus the harem) wake up in an entirely empty world filled only with light, and there they see MISTER POWERSCALING standing before them. (plus the harem.) He turns around and takes off his helmet and oh my god he's actually the first cat guardian. The rest of the harem are all his daughters. (The main catgirl is the firstborn.) He decided to destroy everything because he thinks that the only way to eliminate all suffering from the world is to kill everyone that could possibly experience it. (There's a term for this philosophy but dammit Google isn't cooperating right now.)
The main character, of course, is like "Uh, that's stupid. I've literally died an infinite number of times and I think that's stupid. If, even after all this, I still want to live, why wouldn't everyone else? You're just a murderer, MISTER POWERSCALING."
And in response MISTER POWERSCALING is like "Damn, you're right dude," and hands over his unlimited power to the MC, with it materializing as an orb of blue light. He then turns to dust because with neither his power nor the MC's love, he cannot exist within the total nothingness that once followed in his wake.
Then everyone celebrates for a bit (There's a dance party in the anime adaptation) and the MC and the main catgirl decide to share MISTER POWERSCALING's power so that no one ends up powerful enough to make the same mistake her father did. They then close their eyes and use it to recreate the world from the beginning.
The final chapter is an epilogue (releasing after a two week break) and oh dang it turns out the MC and the main catgirl did get married and they're living in an old house in the country side together with they're two children running around happily in the wheat fields.
Then the MC dies again (because it's been twenty seconds) and everyone just laughs when he comes back to life a few seconds later. The end.
