In the late 19th century, Don Gallico (Price) is a master at devising magicians' illusions who attempts to break out on his own as Gallico the Great. His first performance is interrupted mid-show by his employer Ross Ormond (Donald Randolph). When starting out, Gallico had signed an exclusive contract with Ormond to provide him with any illusions he created. Ormond has a court order forbidding Gallico from using any illusions he created in his own act. This pushes the already unstable Gallico over the edge and he sets out to get his revenge against those who have wronged him, including Ormond; his ex-wife Claire (Gabor), who is now married to Ormond; and rival magician The Great Rinaldi (John Emery), who is Ormond's business partner and the main beneficiary of Gallico's illusions. A clever young police detective, Lt. Alan Bruce (Patrick O'Neal), uses a new identification technique, fingerprints, to identify the killer.
The Mad Magician contains examples of:
- 3-D Movie: The first movie to be broadcast on television in 3-D.
- Badass Boast: Don Gallico gives one for the furnace in which he intends to dispose of one of his victims (and in which the Great Rinaldi had already met a well-deserved end):Don Gallico: You should have seen how neatly this machine disposed of him! In no time at all, he was a handful of ashes!
- Blackmail Backfire: Rinaldi thinks he can blackmail Gallico into continuing to construct illusions for him by threatening to go to the police with his suspicions that Gallico murdered Ormond. It seems he didn't take into account the depth of Gallico's hate for him, or the implications of Gallico having already murdered once. As Gallico later admits to Lt. Bruce, he'll get the same voltage in the electric chair whether he committed one murder or four.
- Chekhov's Skill: During his magic act at the start of the film, Gallico the Great does his own tricks in the style of The Great Rinaldi—a rival Stage Magician—while dressed as, and sounding like, Rinaldi. This hints at the Master of Disguise abilities he will later use to impersonate Ormond and Rinaldi.
- Deadpan Snarker: During discussion of the realism and safety mechanisms involving the furnace Gallico uses for The Crematorium, the question of what would happen if the Lovely Assistant had fallen out with the performer comes up, and Rinaldi responds in a faux-sinister method:The Great Rinaldi: Ashes to ashes.
- Disposing of a Body: Gallico gets rid of Ormond's body disguising it as an effigy dressed in the opposing team's football uniform and placing it on the bonfire the university is constructing to celebrate their victory. The next day Frank Prentiss reads in the paper how only the shinbones and a section of spine were found in the ashes.
- "Eureka!" Moment: Alice Prentiss experiences her Eureka Moment when her husband is reading her details of an unrelated murder in Fall River, and says that a neighbour saw the killer but he was masked. This makes her realize that the man she thought was Ormond could have been Gallico wearing a Latex Perfection mask.
- Forging Scene: There is a scene where Gallico (a young and surprisingly buff Vincent Price) is working at the forge in his workshop; his shirt open to the waist. His ex-wife Claire (Eva Gabor) finds the sight quite a turn-on.
- Gold Digger: Claire Ormond. She was originally married to Don Gallico, but divorced him and married his boss Ross Ormond when she realized he could furnish her with the wealthy lifestyle she craved. Subsequently, she spends half of each year gallivanting around Europe spending her husband's money. After Gallico murders Ormond, she tries to make up with him again, knowing that she will inherit Ormond's money.
- Hoist by His Own Petard: Gallico falls victim to the crematorium he tries to feed Lt. Bruce into.
- I've Come Too Far: Gallico acknowledges that he's beyond all saving as he prepares to murder Bruce and that when the time comes for the legal system to come down hard on him, he'd rather be electrocuted since he'll be going to hell anyway.Don Gallico: No thank you, I'll take the chair!
- Kill and Replace: Gallico murders Ormond in a fit of rage, and then uses his Master of Disguise abilities to impersonate Ormond so no one will know he is dead. However, after he murders his ex-wife Claire while disguised as Ormond, he has to abandon the Ormond identity as the police now want Ormond for Claire's murder. He then murders The Great Rinaldi and assumes his identity; even performing magic shows in Rinaldi's stead.
- Latex Perfection: Gallico is an expert mask maker and is able to create masks that allow him to impersonate both Ormond and Rinaldi. Only Claire is able to see through the disguise when he is posing as Ormand, and she has the advantage of having been married to both Gallico and Ormond.
- Leg Focus: When Lt. Bruce calls first calls for Karen Lee at the theatre, the doorman describes her as "the pretty little thing with the legs. And don't act like you haven't noticed them". Her legs are on display in her role as Gallico the Great's Lovely Assistant.
- Life's Work Ruined: Ormond rings down the curtain on Don Gallico's debut performance as Stage Magician Gallico the Great. Ormond reveals that the contract Gallico signed means that all of the illusions he create belong to Ormond; even the ones Gallico created on his own time. This means Gallico can never appear on stage and, to add insult to injury, Ormondo sells 'The Lady and the Buzzsaw' illusions—a trick Gallico had spent years perfecting to be the climax of his act—to The Great Rinaldi; a magician Gallico hates.
- Lovely Assistant: Karen works as Gallico the Great's lovely assistant, and is the 'lady' in 'The Lady and the Buzzsaw' illusion. Even after his stage career is derailed, she continues to help him develop new illusions.
- Master of Disguise: He has developed Latex Perfection masks which, along with his skill at mimicry that allows him to imitate their voices, allow him to assume the identities of Ormond and Rinaldi after he murders them; fooling even those who know them.
- Mean Boss: Ross Ormond has Don Gallico locked into an ironclad contract that means that any illusion Gallico invents belongs to Ormond, even if Gallico develops it on his own time. When Gallico tries to break out on his own as a Stage Magician — a long held dream of his — Ormond takes out an injunction preventing from performing because all of the illusions are his intellectual property. This effectively prevents Gallico from ever performing as a magician. That he waited until Gallico was about to perform his greatest illusion before ringing down the curtain seems to be an act of pure spite. He also stole Gallico's wife Claire away from him.
- Murder by Cremation: Gallico creates a miniature crematorium to us in one of his illusions. He later attempts to kill Lt. Bruce by feeding him into it.
- Mystery Writer Detective: Alice Prentiss writes murder mysteries. When a murder happens in her own house, she sets out to solve it. She does a very creditable job, and contributes at least as much to the solution of the crime as Lt. Bruce.
- Never One Murder: Foreshadowed when Gallico and Frank Prentiss are discussing Alice's murder mystery novels, in a conversation that takes place after Gallico's first murder but before his subsequent ones:Frank Prentiss: Here's her latest, Murder Is a Must. Rather good title, don't you think?
Don Gallico: Very good. What does it mean?
Frank Prentiss: Well, it's about a chap who murders a chap, and then, so not to be found out, he has to kill another one and then another one, and, well, that's the way it goes, ad infinitum, ad nauseam.
- Nosy Neighbor: Gallico rents the upper floor of the Prentiss house under the guise of Ormond. Unfortunately for him, Alice Prentiss writes murder mysteries and fancies herself as a Mystery Writer Detective. Her snooping brings Ormond's wife Claire on to the scene, causing Gallico to murder her and abandon his refuge at the Prentiss house.
- Off with His Head!: Gallico murders Ormond by using 'The Lady and the Buzzsaw' illusion to decapitate him.
- Read the Fine Print: Gallico discovers that the contract he signed with Ormond means that all illusions he designs—even those he creates on his own time—are the property of Ormond. Losing the rights to 'The Lady and the Buzzsaw'—the signature illusion he developed to launch his own career as a Stage Magician—is what drives him to snap and murder Ormond.
- Revealing Cover-Up: This exchange concerning the murder of Ormond, though at this point The Great Rinaldi merely suspects Don Gallico's involvement:The Great Rinaldi: You had every reason to hate him. What did you do with him, Gallico? How did you get rid of it? [indicating the retort in the center of the room] Was this thing already built and hidden away, waiting for a job like that?
Don Gallico: Ormond was seen alive two weeks ago. The Prentisses identified him.
The Great Rinaldi: That didn't have to be him.
Don Gallico: Who else could it have been?
The Great Rinaldi: It could have been you, Gallico. From what Ormond told me you are a clever man, developed a wonderful new makeup, something new in the theater. Something that might be used offstage too. Only a man who dreams up illusions would think of such a thing, and only people of the theater would know it was possible.
- Satchel Switcheroo: Karen visits Gallico in his workshop after he has murdered Ormond. She moves his bag off his desk while they are talking. When she leaves, she picks up his bag instead of hers. Unfortunately, Gallico's bag contains Ormond's head. Gallico takes off after her to retrieve his bag, only to discover she has left it in the cab.
- Saw a Woman in Half: Gallico the Great's trademark illusion 'The Lady and the Buzzsaw' is a variation of this, in which he appears to decapitate his Lovely Assistant.
- Stage Magician: Gallico attempts to move on from creating illusions for others to start his own career as a stage magician under the name 'Gallico the Great'. After his dream his ruined, he murders rival magician The Great Rinaldi (one of the people responsible for his downfall) and takes over his career.
- Stolen Credit Backfire: The Great Rinaldi plagiarizes Don Gallico's sensational new illusion which involves a fiery furnace and gets himself burned to death. Ironically, Gallico had warned Rinaldi against plagiarizing this illusion.
- Tranquil Fury: Whenever Don Gallico is pushed to the point of murdering someone, he becomes, very, very calm.
- Woobie, Destroyer of Worlds: Don Gallico. A quiet and peaceful man, his driving ambition is to become a successful Stage Magician. Just as he is in on the cusp of seeing his dream comes true (as in literally standing on stage about to perform his greatest trick for an audience on the edge of their seats), his Mean Boss shuts down the show with an injunction that essentially means Gallico can never perform again, steals his greatest illusion and sells it to Gallico's greatest rival. Then his Gold Digger wife leaves him for the boss. Small wonder he snaps and starts murdering everyone who has wronged him. However, by the end, he is trying to murder his friend Lt. Bruce who is merely doing his job and trying to apprehend a murderer, and admits to Bruce that he now can't stop killing.
- Would Hit a Girl: Gallico responds to his ex-wife's goading by slapping her across the face; albeit very lightly.
- Yank the Dog's Chain: Despite being pushed close to the edge of madness by his corrupt contractor Ross Ormond, Don Gallico is perfectly willing to let the legal system handle their dispute over the rights to his illusions. Then, Ormond decides to taunt Gallico once more about Claire leaving him...
- You're Insane!: Lt. Bruce calls Gallico mad after Gallico tries to feed him into the crematorium. Gallico agrees, but says he would rather go to the electric chair than spend the rest of his life in a straitjacket and a padded cell in an insane asylum.