The story is told in part through the mechanics it has the player engage in. It's a bit reductive to describe that as "a popup book"- this is actually quite an interesting way to deliver a wordless narrative. It's fun, well presented, and worth a look.
As others have mentioned, this game was advertised as having multiplayer support; and then released to GoG without it. I bought it anyway, because the developers said they were working on it, and I thought that, worst case scenario, they'd fix it in a month or so. It has been over 3 months now. It's clear that the GoG release is broken and not a priority to fix. All post release development effort went into Brutal Edition, and now I suspect all development effort is going towards the upcoming console release. I wouldn't be surprised if they never fix the GoG version. I suppose there's a chance that the multiplayer implementation they create for consoles can be reused for GoG as well, but I'm not getting my hopes up. And the thing is, this game wasn't just advertised as 'having multiplayer'. It was advertised as a multiplayer game, through and through. Therefore, i consider the GoG version inherently unplayable. This is a demo at best. Don't buy it. Don't buy it anywhere, not on Steam, not on console, until they fix this, or until they pull it from GoG's store and give us our money back. The fact that they still have the nerve to accept money for this on GoG is disgusting.
You know how some games are just wrong? That's this game in a nutshell. "No game, actually, I clearly was not hit and I shouldn't have died. I have footage, you're wrong." "Actually, game, contrary to what you believe, I did press the button at the right time." These are things you might hear yourself saying while playing this. The last boss is particularly guilty of just being wrong: your super powerful retaliation move must be pressed "just before the enemy's attack connects" (super specific description there dev). The problem with the last boss's final form (yes, this is one of THOSE...) is that the solid parts of her are covered by an imprecise particle effect; you cannot actually see when the attack connects and I wouldn't be surprised to find out that the attack's collision detection is completely random. I'm only half joking. There are many moments in my time playing this game where I felt the established mechanics were ignored, and there aren't any design features implemented to help the player learn from their mistakes. Then again, this is an indie game from a very small team, and in that light it's a very impressive creation. I had fun playing (until I reached the last boss on Normal), and there really aren't enough games that push your reflexes to the limit. If you like that type of game, highly recommended- just be prepared for indie level design and polish.