Unknown opens with an international group of young archeology students sweating on a dig on the island of Gotland, uncovering a Viking fortification dating back over a millennium. They are a fun-loving lot, partying together every night, but the good vibe turns to horror when one of them, twenty-one-year-old Martina Flochten, disappears. Her body is found a short while later, naked, bled out, and hanging from a tree. Her injuries indicate that she is the victim of a ritual killing.

Inspector Anders Knutas investigates Martina's acquaintances. Who was the mysterious lover she was supposed to have been meeting in secret and whom none of her fellow archaeologists have actually seen? What do the marks on Martina's body signify? Is there possibly a connection between Martina's death and the recent and unsolved brutal beheading of a Gotland pony? The pony was also bled out, and its head was missing---until it appears mounted on a stick outside the next victim's house.

Inspector Knutas and his team work feverishly to catch the killer, but before long there are more victims, all of whom have been killed and mutilated the same way.