The movie is all about a boy (Shinichi) who has been invaded by an alien, which he named, Migi. Usually, the aliens would go inside human's brain, and Shinichi was lucky for his brain was not taken over by the alien because he has his headphones worn. Not lucky enough, Shinichi's hand was invaded instead. Migi then explpained why he has to use Shinichi. The alien told him that if one of them dies, one of his kind would also die. At first, the boy was unsure if his going to trust the alien, but he agreed eventually. That is because a parasyte had killed his mother and had her body taken over. Because his has become eager of fighting the aliens.
I have learned in the movie that there will come a time that our loved ones would be gone. That is because in the movie, Shinichi doesn't seem to appreciate what his mother was doing for him. He only realized that when her mother died. So, as long as our loved ones are still with us, we should let them feel that we love them and appreciate their efforts for us. Give time for them, and do not just focus on other things, such as phones and other gadgets.Love them as much as we can. That's all.