The plot concerns actors and directors in the early 1930s who were unable to make the transition from silent films to talkies, and thus turned to making pornography. The film's title takes its name from the double meaning that "insert" both refers to a film technique and sexual intercourse. Inserts was filmed like a one-act stage play on one set and filmed entirely in real time.
Prior to the opening credits we see portions of the stag film that is shot in the course of the movie. Voices are overheard that make it apparent that men and women are watching this in the present day. At the end a man complains that there was no "come shot", something that will later develop into a plot point.
The story takes place in Hollywood in the early 1930s, shortly after the start of the talkie period. A visionary and gifted young Hollywood director known as Boy Wonder (Dreyfuss) has fallen out of favor with the studios. This is ostensibly due to his reluctance to lower his standards or abandon his artistic and experimental style, such as using a hand-held camera, for the sake of churning out lesser quality stag films for easy money, due to his alcoholism and his fear of leaving his house. He works out of his decaying mansion, which is the only one left on a street being turned into a freeway.
On the morning of this particular shoot, a heroin-addicted waitress named Harlene (Cartwright) arrives. Harlene was once a well-known and respected star during the silent film era, and she too is reluctant to join the ranks of the "talkies" due in part to her unappealing, high-pitched squeaky voice. She is now the star in the first of his six-picture deal. She prepares and shoots heroin while Boy Wonder drinks heavily during a conversation about the changing times in Hollywood.
An actor called Rex the Wonder Dog (Stephen Davies) soon arrives, wearing a white suit with grass stains on his knees, having just come from his job working for a mortician. During his introduction, Rex gullibly believes a man from a studio who that says that he will put him in the mainstream talkies, and has an appointment to meet him in his hotel room later that same day.
Boy Wonder awkwardly attempts to make an artistic film using an actress under the influence of heroin and an actor who becomes increasingly frustrated with the director and all of his poetic talk, much of which he admits he doesn't understand. The scene goes wrong when Rex gets out of control during the action and Boy Wonder needs to smash a wine bottle over his head to get him to stop.
Just then Big Mac (Hoskins), a porno film producer, enters the scene. He has small heroin packets in his jacket pocket, an unlit cigar in his mouth, wads of money for Rex and a pretty wannabe actress named Cathy Cake (Harper) hanging on his arm. Harlene takes her payment in heroin and soon dies from an overdose in an upstairs bedroom. Rex finds the dead body, and everyone is terribly upset over this turn of events. Boy Wonder talks about continuing his film, but Rex refuses to perform with a dead woman.
Big Mac offers Rex a part in a mainstream movie in order to convince Rex to help him bury the body and, while the two are away, Cathy and Boy Wonder develop a chemistry that eventually leads to another ironic high point in the film. Boy Wonder offers to film Cathy for insert shots of her nude body to double for the late Harlene. At first, Cathy refuses to undress, but when she does, she soon becomes aroused by Boy Wonder filming her. After a while, he makes love to her believing he has found something of a soulmate, but she is disappointed when she learns the camera was off. Boy Wonder's sexual experience with Cathy marked the end of his longstanding problem with impotence, which was evidently related to his emotional problems.
Boy Wonder quickly realizes that this romantic encounter was simply a ploy to get her into the film, and that she has used and directed him the way he used and directed her. Big Mac and Rex return to find both of them half naked. In a jealous rage, Big Mac ends his six-picture stag film contract with Boy Wonder, who by this time is completely drunk. Rex beats up Boy Wonder in retribution for hitting him earlier with the wine bottle by doing likewise. Big Mac takes the film reel that Boy Wonder used and leaves with Rex and Cathy. After Boy Wonder is left alone in his home, a man knocks at the door. This is presumably Clark Gable, a then little-known actor who had been said to be intending to call on Boy Wonder about a film project. Boy Wonder will not answer the door, and after a short time the unseen man leaves. The end of the film finds Boy Wonder alone in his spacious living room, sitting in the same place where the film began; playing piano and singing, pondering what he'll eat for lunch.
This last remark brings home the fact that, while a great deal has occurred in the course of the film, the movie was shot in real time.Richard Dreyfuss as Boy Wonder
Jessica Harper as Cathy Cake
Veronica Cartwright as Harlene
Bob Hoskins as Big Mac
Stephen Davies as Rex
Roger Ebert gave Inserts 2.5 stars out of a possible 4, writing that the film's dialogue was stilted and the setting not entirely convincing, but that Dreyfuss and Cartwright gave effective performances and the film "has a certain quirky charm."