Grimm was an ordinary boy until one fateful Halloween when his life changed forever. He saw something he wasn't supposed to see, a portal into another dimension. Once he crossed over something dark and evil attached itself to Grimm. After that Halloween night, Grimm noticed something was different about him, but he couldn't quite place it. One night Grimm's parents had a huge fight, and this awoke something deep inside of Grimm. After his parents died, and the house they lived in burned down in a mysterious accident, Grimm and his two sisters, Danica and Nikki, moved from foster home to foster home. Each year something continued to grow inside of Grimm, and it was getting harder and harder to control. Grimm began to retreat inside himself, until he met his best friend Amelia at school. That changed Grimm for the better, he began to open up, enjoy life, and make new friendships that would last lifetime. The evil inside of him seemed to have been pushed back for the time being, until another tragic event caused Grimm and his family to move away to Boston.

Five years later Grimm has become a hitman of sorts, using the abilities of monstrosity inside of him to carry out his work. Grimm is not a hero; he's just cleaning up the mess left by society. Grimm only cares about taking care of his two younger sisters. Then one day Grimm gets a text from an old friend for a job. During the job something awoke within him yet again, putting into motion the evil inside of him, plan. There's a demon from another dimension, and a powerful one at that, inside of Grimm. It feeds on his negative emotions, and in return Grimm can use its powers.

Now an ancient evil awakens from its slumber and has its sights on Earth. It's up to Grimm to reunite with his old friends in order to save the world, using the demon's gifts to unlock their potential and make a stand against evil. Grimm will stop at nothing to save not only his family, but the world in which they live.