A skeptical police inspector investigating mysterious blood-loss deaths in Kleinschloss. After wrongly suspecting Herman Gleib, he uncovers Dr. von Niemann's crimes, infiltrates the lab disguised as a servant, and kills the mad scientist.
Their romantic bond becomes endangered when Ruth is captured and Karl confronts Otto.
Ruth Bertin is described as Karl Brettschneider's love interest and lives in the same household while he investigates the deaths.
Their apparent investigative cooperation conceals Otto's attempt to eliminate the skeptical inspector.
Otto discusses the vampire deaths with Brettschneider, advises him to take sleeping pills, and later attempts to drain his blood.
Kringen's testimony and death intensify the inspector's struggle to explain the blood-loss cases.
Kringen reports that he was attacked by a vampire, and his later death becomes part of Brettschneider's investigation.
Martha's death provides a central case in Karl's investigation.
Brettschneider investigates Martha's death and examines the evidence linking her death to the previous blood-loss cases.
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