Rhinoceros opens in a provincial town square after church on Sunday. In a grocery and a café, the mundane squabbles of bourgeois life fill the air. Jean, an upright, no-nonsense, cultured young man, chastises his slovenly, aimless friend, Berenger, for his lateness in meeting him, although Jean has only just arrived as well. Berenger thirsts for an alcoholic drink, and Jean upbraids Berenger's hungover appearance, giving him a comb and a tie. Berenger justifies his drinking as a necessary escape from the boredom of life, especially his dreary work. Jean stresses the need for will-power; he alludes to himself as the "superior man who fulfills his duty."
The sound of a distant trumpeting interrupts the men's conversation. The sound intensifies until all characters on-stage—including a Waitress, the Grocer, the Grocer's Wife, and a Logician—see a rhinoceros off-stage and exclaim their shock. While the rhino rages nearby and the townspeople continue expressing amazement, Berenger remains unaffected.
Berenger orders alcoholic drinks for himself and Jean. Jean presses him to see what he thinks of the rhinoceros, but Berenger cares little about the disturbance. The waitress brings the drinks, and Jean again chastises his friend for drinking at noon. Berenger lies and says he ordered water and the waitress made a mistake. At another table, the Logician explains to an Old Gentleman what a syllogism is (a three-part logical statement with a main proposition, a secondary one, and a conclusion). Jean accuses Berenger of day-dreaming for lack of interest, and Berenger proclaims, "Life is a dream." Berenger tiredly comes up with a number of meager explanations for the rhino's appearance. Jean angrily refutes these and reproaches Berenger for mocking him. Berenger denies this, but lets Jean bully him; he soon accepts Jean's opinions of the rhino and agrees to abstain from liquor.
Daisy, the pretty typist from Berenger's office, passes by the men. Berenger likes her, and in his nervousness spills his drink on Jean. Berenger explains in greater depth why he drinks: when sober, he doesn't recognize himself, but when drunk, he can escape and then identify himself. While Jean lectures Berenger about strength and will-power, the Logician gives a long-winded and eventually incorrect example of a syllogism to the Old Gentleman that concerns cats and paws. Jean refutes Berenger's further descriptions of his alienated misery, labeling them contradictory.
Ionesco explodes a number of profound ideas on to the stage, most of which are situated in the existentialist philosophy of Soren Kierkegaard, Jean-Paul Sartre, and others. The concepts of free will and responsibility are introduced and defined here. Jean is a paragon of will, of having the power to shape oneself according to one's desires. Berenger is his opposite, an alcoholic slacker who cannot even be roused by the unusual appearance of a rhinoceros. Berenger evades responsibility and himself, as is most saliently demonstrated in his attitudes toward alcohol: he lies about ordering liquor and he drinks to escape himself. Yet responsibility is not such a clear-cut issue; while Berenger arrives late to meet Jean, so does Jean. The latter, however, finds a way to justify it.
Ionesco said he wrote the play as a response to the widespread conversion of supposedly free-thinking humans to fascist ideals before and during World War II. Jean's reference to himself as the "superior man" borrows from Friedrich Nietzsche's vision of a "super-man" who is beyond conventional human morality. This super-man, Nietzsche believed, would lead the world. (The concept of a man above morality was critiqued in Fyodor Dostoevsky's Crime and Punishment.) Adolf Hitler exploited (and abused) Nietzsche's ideas heavily in convincing Germans that the Aryans were a master race whose destiny was to control the world. Ionesco's contribution to understanding how millions were swayed is focused in his dissection of a collective consciousness (later referred to in the play as "collective psychosis"). Ionesco posits the existence of a universal mentality that compromises the individual mind. These minds, as Berenger's does in this scene, evade responsibility and willful choice. They allow external ideas to enter without an internal check; as Jean says of Berenger, "There are certain things which enter the minds of even people without one." For Berenger, alcohol is his means for mental escapism, and the false sense of identity that alcohol confers upon him suggests why the ensuing rhinoceros-metamorphoses (and, by symbolic extension, conversions to fascism) are so seductive. Escaping oneself, or belonging to another group, Berenger implies, somehow allows the individual to feel as if he is more himself, a better, stronger, potential self. Still, the benefits of collective consciousness are given their due here; the newly unified community comes together to discuss the rhino.
In a world in which the atrocities of fascism can take hold of human emotions, Ionesco classifies logic as absurd and inexplicable, beyond human rationality. The Logician is parodied for his comic missteps in proving even a simple syllogism; when the Old Gentleman asks if, according to the syllogism, his dog must be a cat, the Logician replies: "Logically, yes. But the contrary is also true." Ionesco further demonstrates the inapplicability of logic to human emotion as he cross-cuts dialogue between the Logician's proof and Berenger's fumbling attempts to provide some coherent reason for his unhappiness. Several key lines assail the inconsistency of logic. Berenger's claim that "Life is a dream" points to life as an absurd undertaking that follows the fractured logic of a dream. Moreover, it accords with his feeling that he leads an unconscious existence with no responsibility (for one has no conscious control over a dream). He later amplifies this doubt as he admits, "I sometimes wonder if I exist myself." His statement contradicts the well-known philosophical premise of 17th-century philosopher Rene Descartes: "I think, therefore I am." For Descartes, the ability to think is the only proof of existence. For Berenger, thought not only fails to certify existence, it even casts doubt upon existence. This doubt articulates the foundation of existentialist philosophy, the formula "existence precedes essence." This important dictum of Kierkegaard's states that humans are born ("existence") before they gain any soul or meaning in life ("essence"). As Berenger (and Ionesco) sees it, neither physical nor even mental existence is enough to count for true existence. Although he does not yet know it, he needs a willful life of responsibility committed to something significant (this will become apparent later in the play).
Ionesco makes plausible the eventual mass transformation into rhinoceroses through two specific stagecraft techniques in this first part. He subtly introduces the first rhino into the play, allowing the sound of the beast to amplify slowly. The off-stage presence of this first rhino piques the audience's interest as well and keeps its existence (or stage existence, at least) in doubt. Ionesco also plants the seeds of collective consciousness in this first scene through dialogue devices; during and after the rhino's appearance, the characters all exclaim the same things ("Oh, a rhinoceros!" or "Well, of all things!") at nearly the same time. Furthermore, two separate, simultaneous dialogues—between Jean and Berenger and between the Logician and the Old Gentleman—discuss similar ideas, sometimes even using the same exact language.