Earth is threatened by an incredible danger - an alien invasion. But it just doesn't happen the way it is usually shown in science fiction films. After all, aliens are small worms that penetrate the human body and take total control over it. Once in the brain of the victim, they erased the individuality of their new "abode" and completely controlled the body.
Shinichi Izumi was also attacked by the alien. But for some reason, the parasite could not get to the guy's brain and got stuck in his right arm. The teenager called him “Right” and was even able to find a common language with an unexpected intruder. They made an agreement - the guy eats for two, and the Right one helps him in fights. After all, the aliens realized that Shinichi knows their secret, which means they will definitely come after him.
The aliens are still too weak for a full-scale invasion. And humanity still has a chance to cope with them. It's good that the Right is an egoist who only cares about his existence. Therefore, he agreed without any problems to fight with bodies controlled by enemies, protecting his symbiont.