The setting is Prague during 1934. Irena Borotyn and her fiance Fedor have returned home to her family estate, so they can be married. Unfortunately, soon after their arrival, Irena's father Sir Karell is found murdered, with two puncture wounds on his neck and with his body completely drained of blood. The local Agent Mulder doctor immediately diagnoses a vampire attack, and the townspeople blame one "Count Mora" (Lugosi), believed to be the local vampire, and his vampire daughter Luna. The wedding is postponed, and Irena goes to live with her guardian, her late father's close friend Baron Otto (Jean Hersholt).
A year passes, and once again it's time for Irena and Fedor to get married. Unfortunately, Fedor is attacked by a vampire, and Irena is attacked by Count Mora's daughter Luna. The good guys call in Professor Zelen, an expert on vampire lore (Barrymore), who tells them that Mora and Luna must be destroyed during daylight hours. But the vampire duo aren't going to go away so easily...and in the meantime, Sir Karell's coffin is empty...
Mark of the Vampire was an attempt by MGM to capitalize on the vampire craze started by Dracula, by hiring Bela Lugosi to put on his cape again. It is a remake of Lon Chaney silent vampire movie London After Midnight, also directed by Browning, and infamous as one of the most famous lost film of the silent movie era.
- Artistic License Biology: Baron Otto supposedly drained Sir Karell's blood with—a cup. The human body contains five liters (slightly over one gallon) of blood.
- Cat Scare: The doctor and the inspector are freaked out when a suit of armor starts rattling on its own...but it's a cat that crawled into the helmet.
- Cobweb of Disuse: Although this film is a remake of London After Midnight it was obviously meant to evoke the 1931 Dracula. So the Borotyn mansion is covered with cobwebs and dust, as Dracula's castle was.
- The Ending Changes Everything: See Twist Ending below.
- Fate Worse than Death: Prof. Zelen says this about becoming a vampire, saying "We must all die. There's nothing terrible about death." But existence as a vampire, that's bad.
- Have a Gay Old Time: "And tonight you're gay and cheerful."
- High Collar of Doom: What, was Lugosi going to play a vampire and not wear his collar up?
- Identical Stranger: Conveniently, the scammers are able to find an actor who is the spitting image of Sir Karell.
- Large Ham: Lionel Barrymore is very aware that he is in a very silly movie.
- Lesbian Vampire: It was 1935, a year after the imposition of The Hays Code, so they couldn't be too obvious about this. But it's noteworthy that it's Luna, not Count Mora, who attacks Irena twice.
- Ominous Fog: The cemetery is appropriately fog-bound when the gang goes to Sir Karell's tomb and finds it empty.
- Ominous Pipe Organ: After being turned into a vampire Sir Karell passes the time by playing the organ. (In hindsight this is probably part of the scam to make Baron Otto nervous.)
- Our Vampires Are Different: Interestingly, Zelen says they aren't killed by the whole stake-through-the-heart deal. No, vampires have to be decapitated, with the anti-vampire plant "bat thorn" placed between the head and the body.
- Plot Hole: There are several events that don't make a lot of sense given the Twist Ending.
- The townspeople cast suspicion on Count Mora and his daughter Luna, who are apparently the town vampires or something. Was the whole town in on the scam?
- Who attacked Fedor?
- There are multiple scenes when the players are staying in character when Baron Otto is not around, like an elaborate shot when Luna descends from the second floor to the first by sprouting bat wings and flying down.
- If Zelen could get Otto to expose himself by some judicious use of hypnosis, why was the whole vampire charade necessary?
- The Reveal: See Twist Ending below.
- Satellite Character: Besides Mora, Luna, and Sir Karell, there's a fourth vampire, a male character, who does nothing.
- Twist Ending: There are no vampires. Baron Otto killed Sir Karell because he wanted Irena for himself. "Count Mora" and "Luna" are actors hired by Inspector Neumann and "Professor" Zelen (actually chief inspector of Prague police) to help trick Otto into a confession. Irena is part of the plot and faked hearing her father's voice and being attacked by Luna. (This ending has been controversial ever since. Apparently Lugosi was horrified by the twist.)
- The Voiceless: Unlike Dracula, which got a lot of mileage out of Lugosi's accent, in this film both Mora and Luna are silent vampires. Until the Twist Ending, that is.
- Voluntary Shapeshifting: In another bit stolen from the 1931 Dracula, Count Mora and Luna change into bats.