"King of Thorn" is a science fiction anime series based on the manga by Yuji Iwahara. The story is set in a near-future world where a deadly virus known as the Medusa has spread globally, causing its victims to turn to stone. In response, a select group of individuals is chosen to undergo cryogenic sleep in a facility called the Cold Sleep Capsule Center, located in a remote castle in Scotland. The hope is that they will awaken in a future where a cure has been found.

The protagonist, Kasumi Ishiki, is a young woman who volunteers for the program after her twin sister, Shizuku, succumbs to the virus. Alongside her are other participants, including Marco Owen, a former soldier, and Timothy Wallace, a scientist. The group is placed into suspended animation, but when they awaken, they find the facility overrun by monstrous creatures and the world outside drastically changed. The castle is now a labyrinth of thorny vines, and the survivors must navigate this hostile environment while uncovering the truth behind their situation.

As the story progresses, it becomes clear that the Medusa virus and the events surrounding the cryogenic project are more complex than initially understood. The characters face not only external threats from the creatures but also internal conflicts and psychological challenges. Flashbacks and fragmented memories reveal deeper connections between the characters and the virus, adding layers of mystery and intrigue. The narrative explores themes of survival, identity, and the consequences of scientific experimentation.

The plot takes several twists as the group discovers that time has passed differently for them, and the world they knew no longer exists. The true nature of the Medusa virus, the purpose of the cryogenic project, and the origins of the monstrous creatures are gradually unveiled, leading to a climactic confrontation. The series blends elements of horror, mystery, and science fiction, creating a tense and engaging story that keeps viewers guessing until the end.