It follows New York newspaperman Lee Taylor (Tracy) as he covers the investigation of a series of full-moon murders in which victims are strangled, cut with a scalpel, and partially cannibalized. Evidence points to the Academy of Surgical Research, where five scientists — including the director, Dr. Xavier (Atwill) — have remained to continue their research while the students are on holiday.
Desperate to protect the reputation of his institution, Xavier strikes a bargain with the police: he will bring the suspects to his Long Island mansion and monitor their vital signs as they watch staged reenactments of the crimes. If he can discover the killer within 48 hours, his name and the Academy's will not be revealed to the newspapers in connection with the murders.
Lee will have nothing of this. He reports the story anyway, and secretly follows the investigation right to the mansion. Too bad Xavier's daughter, Joan (Wray), already recognizes him...
Doctor X was shot in both black-and-white and Technicolor; it was one of the last films to use the two-strip Technicolor process. The color version was believed to be lost until 1978, when a print was discovered in Jack Warner's personal collection. This, in its restored form, is now considered the definitive print, and for over 30 years was the only version of the film available until an HD restoration of the black-and-white version was included on the 2021 Blu-ray release as a bonus feature.
- Artificial Limbs: Wells's hand.
- Beach Episode: Lee and Joan have a non-plot-related conversation on the beach in the middle of the film, solely to get in a bit of comic relief (from him) and Fanservice (from her).
- Beat Still, My Heart: Wells's experiment, a heart kept alive in a jar.
- Bully and Wimp Pairing: The servants have shades of this. Otto is a petty sadist who takes every opportunity to scare high-strung Mamie — and then they're called on to reenact the killings together.
- Catchphrase: Lee's "Bad luck!"
- Deadly Prank: Inverted. An Explosive Cigar saves Lee's life by startling the killer at a critical moment.
- Disney Villain Death: Combined, gruesomely, with Kill It with Fire.
- Dolled-Up Installment: The sequel, The Return of Doctor X, was based on an unrelated short story. What ties the two films together is their Applied Phlebotinum: synthetic flesh in the original and synthetic blood in the sequel.
- Electric Joybuzzer: Averted. A handshake buzzer appears as a running gag (and is eventually even used as a weapon), but it's the realistic, mechanical kind.
- For Science!: Part of the killer's motivation.
- GPS Evidence: Police narrow the suspect pool considerably when they discover that the scalpel used in the murders must have come from the Academy of Surgical Research.
- Hollywood Darkness: Averted at least twice: when the lights go out during the first reenactment, and in The Tag.
- I Need a Freaking Drink: After the housemaid sees a bad omen in her tea leaves, she refills the teacup with gin. From a bottle she keeps under her pillow.
- I'm a Humanitarian: The killer, seemingly.
- Instant Sedation: In the form of knockout gas, used on Lee while he is in the room of skeletons.
- Intrepid Reporter: A variation. Lee pursues his story relentlessly, but otherwise doesn't fit the character type: he's a skittish Non-Action Guy who only stays on the case because of pressure from Da Editor.
- Lovable Coward: Lee.
- Mad Scientist: All of the doctors have elements of this, but the killer embodies the trope perfectly.
- No Party Like a Donner Party: Implied to be part of two of the doctors' shared backstory.
- Obfuscating Disability: The killer embodies this trope in spirit: even though he really does have only one hand, he is able to easily produce a functional, living replacement.
- Porn Stash: Haines's magazine of "French art," hidden in a book.
- Red Herring: Doctor X, X being the unknown, is likely to make viewers think the killer is Xavier, whose name starts with an X.
- Red Right Hand: Three of the five doctors have physical oddities that could potentially mark them as villainous: Haines wears a goatee, Rowitz has a Dueling Scar and a dark monocle, and Duke uses a wheelchair and crutches. Of the other two, Xavier is ordinary-looking and Wells's one-handedness actually exempts him from suspicion.
- Running Gag: The hand buzzer.
- Scary Flashlight Face: Used several times (without actual flashlight), especially when Otto is in the shot.
- Sliding Scale of Comedy and Horror: Leans heavily toward the comedic side, except during the climax.
- Spanner in the Works: Lee, during the second reenactment.
- Spiritual Predecessor: To Mystery of the Wax Museum.
- Techno Babble: Xavier's explanation of the stress-detecting device:Gentlemen, I am now turning on the hundred-milliampere high-frequency coil. Your pulses are connected with the magnetic rotators, and each variation of your heartbeat reaction is amplified four thousand times. The rotor of the electrostatic machine is connected in multiple series with a bank of glass-plate condensors, and the discharge causes irradiations to the thermal tubes, which, in turn, indicate your increased pulse rate and nerve reactions.
- Throw It In: The film is full of fluffed lines and obvious ad-libbing.
- Throwing Off the Disability:
- Duke is briefly able to walk unaided during the first reenactment. This is immediately recognized as a temporary result of extreme stress and has no further impact on the plot.
- The killer restores his missing hand with artificial flesh.
- Trailers Always Lie: The original marketing depicted the film as a comedy-thriller-romance, deliberately concealing the substantial horror element.
- Undercrank: Used, fairly subtly, in the final fight scene.
- Waking Up at the Morgue: Played with. Lee hides in the morgue by posing as a corpse, but then behaves as though he hadn't expected to be there. And where did the toe tag come from?
- Well-Intentioned Extremist: "I'll make a crippled world whole again!"