The plot is that alien creatures - Parasytes - invaded secretly the Earth to fullfill their hunger to human flesh and some came to learn about US. They enter the human body and take contol of the brain and the whole body. As the result of that they can change the body shape - have arms and legs as blades, large sharp teeth and any other features which can help them kill a human and devour them. However some keep their human faces and blend into the society. Why reveal yourself and cause panic? You would get caught. Shinji Izumi is the only one who knows about them as one of them failed to invade the brain and invaded his hand.
The manga isn't drawn very modern but I somehow loved it. In the anime the graphics are modern and characters don't look the same as in the manga. I barely recognised the main character. The reason that I recognised this anime was because of parasytes. Despite the fact that graphics changed, they are still good! The changes of the body are amazing. Changing the shape of the body is certainly difficult to animate as a human form changes to something really bizarre. Also - finally no censoring. I hope to see the dramatic scenes in anime version and I look up to the gore. Additionally, the opening is amazing with some metal parts of the song. It suits the anime so much!! The music is well chosen.
In my point of view, I think that anime normally follows the manga, however ther are some changes for example modern technology such as tablets. In 1995 and below nothing like this existed therefore they changed a bit for the anime to be base on our modern life. Anyhow, after watching the first episode, I'm glad to say; you have to watch it!