Can anything stop a creature that won't stop growing?

Sam Taylor just wants a friend. Is that too much to ask? His only mistake is finding that friend in Felix, a lab mouse that Sam rescues from the top-secret facility where he works as a janitor. Shortly after his rescue, the mouse begins to change, to swell. There's something new growing underneath Felix's fur. Growing very fast.

Holed up in a roadside bar, four survivors—a woman who's lost everything, her drug dealer, a tribal police officer, and a professional gambler—are all that stand between the rampaging beast and the city of Las Vegas. But as the monster keeps growing—and eating—how long until it's able to topple the walls protecting them?