Shela: Be grateful for that. Not remembering is the only thing keeping you alive!
David: But why?
Shela: Because you know something you shouldnt about him. But also, you have something he needs. Thats why hes taking a chance on keeping you alive a little longer.
David: Ill have to write him a thank-you note.
Peck plays David Stillwell, a cost accountant—or is he? His memory suddenly becomes mixed up, starting from the moment when Unidyne headquarters, the office building he works at, has a power blackout—which happens just before Wealthy Philanthropist Charles Stewart Calvin (Walter Abel) plunges to his doom from an upper floor. David is bewildered that he can't recall even simple particulars of his job, and places he does remember don't exist. Believing he has some type of amnesia, he tries to backtrack what happened before the blackout. Aided by rookie detective Ted Caselle (Matthau) and confused by Shela (Baker), a woman he doesn't recognize but who claims to be an old girlfriend, David tries to make sense of the inconsistent facts.
Meanwhile, people around David are being murdered—and he may well be next. He's being stalked by hitmen working for a powerful, mysterious figure called "The Major" (Leif Erickson). Shela says that David has something the Major wants, but he can't remember either the Major (who is allegedly his boss) or what the sought-after item is. And Calvin's death has something to do with the mystery. Can David regain his memory before he becomes the next victim?
- Amnesiac Hero: David suffers from Identity Amnesia brought on by a traumatic event which he cannot remember and hence tries to uncover. He even hires a Private Detective to help him find out who he is really is.
- Bait-and-Switch Gunshot: During the Russian Roulette scene, as we expect Willard to pull the trigger on David for the third time, we hear a shot and see the baddie collapse. Then the camera shows Shela as she holds the smoking gun. Depicted on the lobby card above.
- Big Applesauce: Mirage was filmed on location in New York City, and is a worthwhile guide to what NYC looked like in the mid-60s.
- Broken Pedestal: David admires Charles Calvin at first, then learns that his mentor's foundation has illegally been working with the Major and Unidyne on the deadly formula. Their subsequent argument is the indirect cause of Calvin's death.
- Celebrity Paradox: Averted. According to the press booklet, the filmmakers removed an ad for a TV showing of the 1956 Moby-Dick film (which starred Gregory Peck as Captain Ahab) from a subway car.
- Corrupt Corporate Executive: What the Major turns out to be. He's the head of Unidyne, and he's willing to have people murdered, threatened and beaten to get what he wants.
- Bad Boss: Considering that David is both one of the Major's victims and his employee...
- Dead Star Walking: Walter Matthau's character surprisingly dies at the end of the second act.
- Dehumanization: The argument that begins the story is all about it.David: Were being turned into statistics, case histories, and percentage points, all in the name of progress! Whatever happened to human beings?Charles: Is that what you want to see, David? Human beings? Come here, David. Look at them. (Gestures at a New York City street) Do they look like human beings, or ants? You're quite right, David, they are statistics. But I didnt do it to them. Im not responsible.
- Destination Defenestration: Charles Calvin's death is an example.
- Driven to Suicide: A newspaper headline says that Calvin's death is an apparent suicide, and a background character also assumes that this is the case ("If I had the guts to step out that window, I'd have the guts to go on living"). It's not true.
- Evil Old Folks: Bo (House Jameson), one of the Major's assassins. Jameson was in his early 60s at the time of filming.
- Executive Suite Fight: A variation: the film's climax, with the No-Holds-Barred Beatdown and Russian Roulette scenes, takes place in the Major's secret penthouse.
- Fail O Sucky Name: Joe Turtle (Neil Fitzgerald), the Unidyne receptionist. He likes David because "Youre the only man in this whole building who can say my name without making it sound like a joke." Also see the spoiler text under Only Known by Their Nickname.
- Faux Affably Evil: Lester, who casually converses and jokes with David while holding a gun on him the entire time.
- Four Eyes, Zero Soul: Willard is a brutal assassin whose distinguishing characteristic is his wire rim glasses.
- HeelFace Turn: Happens at the climax when Josephson turns against his boss, Major Crawford. David's Rousing Speech while he's dithering helps: "Commit, Josephson! If you're not committed to anything, you're just taking up space!"
- High-Class Call Girl: It's implied that Shela is a kept woman for various Unidyne executives, although her feelings for David are genuine.
- I Need a Freaking Drink: David suggests having a drink after he and Caselle couldn't find his office nor Joe Turtle at his work place.
- It Was Here, I Swear!: When David tries to show Caselle Lester's gun and hat, which he stored up in a cupboard, they are gone. So are all the papers in his valise. In a variation of the trope, David also finds that his supposedly empty refrigerator is now full.
- Last-Name Basis: Caselle and Josephson are rarely referred to by their first names.
- Line-of-Sight Name: David unconsciously picks the profession of a cost accountant as his job because that's what he compared himself with shortly before the traumatic incident. As David says to Calvin: "Those people aren't even ants to [the Major]—they're articles of commerce! That man computes human life in terms of dollars and cents. Hes made you his prize salesman, and Im the cost accountant trying to cut down his overhead with what you and he call 'progress!'"
- MacGuffin: The secret that's hidden somewhere in David's missing memories. Whatever it is, the Major is willing to kill for it. It turns out to be a formula for neutralizing radiation, which not only has peaceful purposes but would make waging nuclear war a lot more convenient. David is actually the physio-chemist who invented the formula, and his moral qualms about doing so are central to the plot.
- Missing Floor: The Unidyne building has several of them.
- On his way down the stairs in the beginning, David notices that the 13th floor is missing. Shela remarks that this is because the natives are superstitious. When David chases Shela down the stairs, he goes down four sub-basements to emerge in the boiler room. Later those floors aren't there. He has amnesia but doesn't know it yet. Those four floors below ground are actually from Garrison Labs in California, where David developed his anti-radiation formula.
- Later, David uses a special key to get to the Major's office on the otherwise inaccessible 65th floor. For what it's worth, 13 x 5 = 65.
- My God, What Have I Done?: David has two moments like this, the first when he realizes that his formula would make nuclear war easier, and the second when Charles Calvin falls to his death while trying to retrieve the formula when David destroys it, although he eventually realizes that this wasn't his fault. Both are important to the overall plot.
- New York City Subway: David rides it in one brief scene. See Celebrity Paradox above.
- No-Holds-Barred Beatdown: Willard gives one to David just before the Russian Roulette scene, stating that he "owes" David for successfully fighting him off earlier.
- Offscreen Moment of Awesome: When Lester is holding David hostage, he makes himself at home in the apartment by watching Professional Wrestling. As the two men fight, the camera shows the TV screen—then turns to David (who presumably took tips from the wrestlers) standing over the defeated Lester.
- Once More, with Clarity!: On several occasions, David has flashbacks to a scene of two men standing in a park under a tree. It's only at his second visit to Broden that he grasps the full context. It was himself and Charles Calvin discussing the impact of his formula in front of his working place, the radiation lab in California.
- One Name Only: Shela, Willard and Lester.
- Only Known by Their Nickname: The Major's real name is rarely used. If your real name was Crawford Gilcuddy, you probably wouldn't use it often either.
- Police are Useless: David goes to the police, only to leave in frustration when he's unable to remember enough personal information (starting with his "date and place of birth") to even fill out the paperwork.
- Posthumous Character: Charles Calvin has just died at the beginning of the film, but he appears in several flashbacks.
- Private Detective: Ted Caselle. David is his first and last client.
- Pursued Protagonist: Throughout the movie, David is being followed by murderous mobsters, but due to his amnesia he can't figure out why they are after him.
- Russian Roulette: Used on David to force him to write down his formula. Shela intervenes before the third bullet can be fired.
- Shoot the Hostage: Willard shoots his partner Lester without hesitation when Stillwell tries to use the latter as a Human Shield.
- The Shrink: Dr. Augustus J. Broden (Robert H. Harris), the psychiatrist David consults for help.
- Those Two Bad Guys: Willard (George Kennedy) and Lester (Jack Weston), the two assassins working for the Major who get the most screen time. They also work together a lot.
- Tied Up on the Phone: How Caselle is murdered.
- Token Minority: Lt. Franken (Hari Rhodes), the police offer David seeks help from, is the only non-white character with a speaking role in the entire film.
- Took a Level in Badass: David is forced to do this in order to fight off the Major's henchmen.
- A Tragedy of Impulsiveness: The plot centers around one. When David learns that his hero Charles Calvin, a supposed peace activist, is working with the Major on the potentially dangerous anti-radiation formula, they have a fateful argument which ends with David burning the formula's only copy and throwing it out Charles' open office window. Charles leans too far out the window to retrieve the paper and falls to his death—and David's resulting shock, trauma and guilt trigger his amnesia.
- Train Escape: After their encounter in the park, David escapes the henchmen by quickly getting onto a bus that rushes off before his pursuers arrive at the station.
- Trash the Set: After finding Caselle lying strangled on his desk, David has a Heroic BSoD and subsequently trashes the office.
- Witty Banter: David and Shela engage in it, especially before David realizes how much danger he's in.
- World of Snark: Although Caselle has most of the one-liners, several other characters get off a good zinger on occasion.
- Yes-Man: Sylvester Josephson (Kevin McCarthy's character) has spent most of his career being one to the Major.