Former Olympic boxer Jim Ratzkirk used to be a contender. Keywords, “used to”. Now he’s a washed up nobody with a rundown gym in the middle of a bad part of town. To make ends meet in his failing enterprise, he’s rented out the basement to a murderous scientist who may or may not be cooking meth down there. Things haven’t been going well for good ol’ Jimbo.

Just when he thought things couldn't get worse, Jim's decision to try an experimental, rat-tested youth rejuvenating steroid gives him a tantalizing taste of his old vigor. It's an addictive taste with some unsettling side effects. It's not long before Jim's used up the last of the sample formula and in a desperate fit of 'roid' rage, drinks the blood directly from a few infected lab rats. Big mistake.

The next day, while he is pummeling a couple of local thugs in his boxing ring, Jim triggers a chemical reaction in his body that transforms him into a ravenous, man-sized rat. Trapped like live bait in the gym, Jim's niece and a handful of survivors must try to reason with the man Jim was or destroy the thing he's become before all are killed.