When his brother goes missing on a remote hiking trail, Jack Rollings goes in search of him but soon finds himself in a dangerous game of cat and mouse with a creature thought only to be that of legend.

Students on summer break are exposed to a deadly virus that is spread rapidly through direct human contact. The infected become enslaved by the invading "swarm" intelligence and driven by an insatiable appetite to consume human flesh. Returning home, the students spread the infection to their fellow classmates and other unsuspecting townspeople.