A group of friends are plagued by recurring nightmares set in the Middle Ages. In an attempt to bring peace to their nights, they decide to undergo a psychic seance. They soon find themselves in an unlikely Middle Ages, battling a swarm of accidentally released killer horseflies. To defeat the pesky insects, they must find and smoke the dried leaves of a magical herb.

A group of friends on vacation are attacked by a swarm of killer horseflies. The situation worsens because the flies are attracted to blood and attack their victims, causing them to implode or cause serious injury to feed. The friends must use large amounts of cannabis to repel the insects, discovering that the THC in marijuana is the only substance capable of stopping their rampage.