The beginning of the manga starts with our main character in a room hiding for his life. Our main character is a bigger guy that is short, weak, and has nothing really going for him. He is hiding because outside is the zombie apocalypse and the world is falling into ruin. He ends up letting 3 guys into the place where he is hiding because he is friendly but they are total scumbags and kick him out to fend for himself.
Our main character then gains the system which says it will help him survive and become stronger. After killing a couple zombies and seeing these orbs of light fly toward him he realizes that every time he kills a zombie he absorbs some of their strength. His goal now that he is stronger is to go and save a woman who I believe is a professor or some faculty to a university.
On the way he meets another girl trapped in a building who was always mean to him but now he is stronger she tries to screw him over and get him eaten by zombies however that plan does not work. It is later revealed that this girl's father has power in the military. He saves a different girl other than the one that tries to screw him over and decides it is time to go and save his crush.
He goes to save his crush with the woman he saved following behind him. After he and his crush he push out the building, or something along those lines, he keeps killing zombies and the system says to him that it is time for him to evolve. However to evolve he needs someone from the opposite sex that is infected, which turns out to be the woman he has a crush on. He evolves and through that process also cures her of the zombie disease.