The very intro of the game has the main character being pinned at a table, about to be dissected by a scientist as the whimsical "MAD RAT, ALIVE?" plays and the game asks you questions about your worldview. By the last question, the scientist has cut the poor rat's stomach open, with his intestines bulging out and blood seeping from the cut.
After supposedly dying, the titular Mad Rat does come back to life... in the form of a stitched-together zombie. There's a giant hole where his chest and stomach should be, with the tips of his ribs showing and surrounding a floating heart.
The Doctor performing these experiments is humongous (not to mention cruel). His entire hand covers Mad Rat at the end of the fourth stage and effortlessly places him back in the cage he just passed through.
The other rats in that cage are not looking that good, either. In contrast to their initial appearances, Mad Rat notices that they look wrong in the head - their eyes point into different directions, and they stutter while acting like nothing is wrong. The screen itself momentarily covers itself in static while replaying their talking animation. The last one Mad Rat talks to has their brain pop out, and Mad Rat has to fight a whole stream of them after that...
The way said brain comes out is also pretty messed up. Instead of just appearing, we're greeted to the amazing view of the rat's head tearing open near the top before its brain seeps out. The fact that the brain is impaled with pins makes it even worse, and it is very clear from the profuse screaming from the rats that this process is extraordinarily painful.
The fact that their brains are given colours similar to the Nightmares, explode by themselves and regrow (sometimes with no period where they're vulnerable), and also leave pools of harmful liquid when they're beat hints that there might be something more going on...
The aftermath seems to be a case of Bloodless Carnage, but the Rat God mentions that Mad Rat saw so much blood. Considering that the zombies are leaking yellow liquid from their mouths...
Even Mad Rat is affected. The Rat God implies that Mad Rat asphyxiated after seeing so much blood.
Nightmares appear to be innocuous at first, they are gaudy and look like they're graffiti come to life. The issue comes from the fact that they appeared in the afterlife training level where dreams are a heavier theme. Once Mad Rat is "alive" again, despite that fact, they keep appearing as enemies for some reason. The boss fight with the Mob Rats above hints that there may or may not be a connection...
The Ghost of Culvert is made up of multiple eyes set in blackness and a horde of sleeping ghostly rats for a head. The ghost is very disquieting and feels more like something to be found in Yomawari than this game.
Stage 3-5 has Mad Rat fight Heart, which exploits his potential of Nightmare Fuel.
Heart somehow drags Mad Rat to a monochrome dust bowl scattered with dead trees. This is later revealed to be the Afterlife's true form, as seen in the last few stages of the game, albeit without the neurons in the sky.
One attack really takes advantage of the fact that Heart only has one eye: the attack surrounds Heart with a red cloud that gives Heart the appearance of an Eldritch Abomination.
A lot of attacks involve blood: blood shots, blood gushers, and blood rubies. Notably, despite blood being normally portrayed with a yellow or pink colour, the blood from Heart is red... excluding a few attacks, where the blood gushers are somehow purple.
Heart can stop time. The music even stops while this happens.
The Cat in Stage 4-1's only visible body parts are his teeth, whiskers, and crazed eyes peering from the shadows. Then its arm and claws peek...
Mad Rat meets other rats who are deluded into thinking that they are getting lots of cheese... before a real cat (from Stage 4-1) eats them, leaving a trail of red blood. Unfortunately, the cat is about to eat Mad Rat as well... had a little human girl not picked Mad Rat out of the way.
Unfortunately, the girl ends up dying in a car crash. All you see is her bloodied hat, more red blood, and her arm. Thankfully, Mad Rat can rewind time.
Mad Rat and Heart piece together the true nature of Mad Rat's journey. Mad Rat was under heavy hallucinations that tricked him into thinking that he killed the scientist and his fellow ratmates, and the ghost of the ratmates were haunting him. In fact, he did not really go back to the laboratory and travel to a dust bowl, but rather went outside to the park. As for the cheese he has been chasing, that was a trap that was meant to lure Mad Rat to a cat that obviously wants to eat Mad Rat. Upon realizing this, Mad Rat falls unconscious and meets the Rat God... who says that Mad Rat ran out of time and was going to die... because a cat is going to eat Mad Rat. Mad Rat cannot move while the Rat God is present. Even so, once the Rat God disappears, Mad Rat find that Heart is gone.
Even if the doctor did vivisect Mad Rat, seeing the dead doctor's arm by a pool of blood can get scary.
Mad Rat left his mark on the world, which means that he is going to get his happy ending, right? Nope. The Rat God, upset that Mad Rat did not get eaten by a cat, rewinds time back to before his vivisection. Heart then reveals that she intends to have Mad Rat live in a "Groundhog Day" Loop until he gives up and lets himself be eaten by a cat.
The next stages are a Dark Reprise of the tutorial stage. Instead of cheese, there are skulls. Platforms covered with red thorns are harmful to Mad Rat. The whole environment becomes bleaker. Mat Rat outright calls the place 'a hellish heaven'.
The last stage reveals the true nature of the Rat God: she is a parasite inside Mad Rat's brain who wants to reproduce by having a cat eat Mad Rat. She doesn't even care about dying in the process, being completely surrendered to instinct.
The Rat God gets arguably even scarier if you know it's based off a very real parasite (Toxoplasma gondii), that really does influence rats' brains to get themselves eaten by cats so it can reproduce.
The Rat God's battle is similarly terrifying. She initially transforms into a quite distorted version of herself, but she can also turn into her true parasite core surrounded by a rock ring in the shape of a fusion between the Sun and the Moon. All the while, she can, among other things, summon blocks that spread 'fire' and a black hole that can suck Mad Rat in.
There is an Uncanny Valley effect from seeing Heart's true form: a cat without a nose or a mouth. This stands in contrast to the cat Mad Rat rescued earlier, which, while hidden in shadows, at least has a complete face.
