A NEET that no one would miss is about to get hit by a truck but a white haired beauty in strange clothes appears in front of him and disintegrates the moving mass of death. Obviously grateful to his savior, the guy tries to thank to her but is instead accosted by her and finds his neck on the sharp end of her glowing sword. Moments later the sword's color shifts from white to green and she puts it away in irritation.
Putting him down, she explains he is caught up in a interdimensional war where souls from earth are being run over by special trucks to be taken to other worlds to fulfill a particular role. While the victims are told they are meant to be heroes who will save the worlds they've been brought or reincarnated into, the forces that sent them there actually intend for them to prepare those worlds for their harvest of souls.
The girl was one such victim but the truth was revealed to her and she joined up with another mysterious force that opposes them. She informs him that they won't stop coming after him now since they've marked him for his latent cheat ability and that she's going to use him as bait to lure out their enemy.
He agrees to this nonsense and brings up how she's going to end up falling for him as per the genre but she coldly denies him that possibility as she loathingly announces that while he may see a beautiful girl in front of him she still considers herself 100% a man on the inside since she still has all her memories of her past life.
Thus the curtains raise on a very chaotic isekai battle harem.