Ollie Henderson is celebrating his birthday with his mom, Rebecca, when on their way home, they are attacked by something in the darkness. Ollie screams for help, trying to hold on to his Mom, but she is ripped from his arms and dragged into Lake Merritt. Ollie’s dad Baldemar, an Oakland Police Officer, arrives on the scene. He searches the area for answers but a visibly shaken Ollie can only utter one word - “Monster.”

Three years later, and the scene still haunts Ollie’s nightmares. He feels his mother’s disappearance is his fault, and is filled with shame that he did not fight to protect her. Determined to make things right, Ollie is constantly searching for his mother, and for clues that might lead him to the mythical Lake Merritt Monster. Along with his friends Ham, App and Heiro, Ollie devises a plan to find and kill the beast, not realizing the cost to his friends and family. It’s only with a little help from a monster hunting Granny that Ollie learns his family’s own legacy with monsters - one that his mother was hoping to pass down to him. Armed with renewed strength, ingenuity and the knowledge of his own role as a next generation monster hunter, Ollie sets off on one last mission to defeat the Monster and find his mother.