Leaving his Frankenstein monster portrayal behind, Boris Karloff shows in this film that he can act and do a good job of it. He plays a Greek General trapped on an island with others fighting the plague.
A young girl, Thea (Ellen Drew) is thought to be a vrykolaka, a harmful undead creature in Greek folklore, by an old peasant woman (Helen Thimig). It has similarities to many different legendary creatures, but is generally equated with the vampire of the folklore of the neighbouring Slavic countries. While the two are very similar, blood-drinking is only marginally associated with the vrykolakas.
The tale is a mixture of folktale and virus, of medicine and miracles, of superstition and madness. Karloff carried the film with marginal assistance from the others. Worth a watch to see his acting.