A group of tourists arrive to see the botanical gardens on a small island off the Italian coast. The only inhabitants of the island are the famous botanist Baron von Weser and his two faithful servants; all of the other residents fled after a series of mysterious deaths. When the car driven by the tourists' guide hits one of the baron's servants, von Weser explains that the death was due to a rare and incurable disease, not the accident. That night, the guide and Cora Robinson, one of the guests, are found murdered with all the blood drained from their bodies.