The play opens on the square of a small, provincial town on a Sunday afternoon. A woman crosses the stage carrying a cat in one hand and a basket of provisions in the other hand. She exits. The Grocer's Wife is onstage, and she remarks that she does not like that woman because she is too stuck up to buy from her anymore. Then the Grocer's wife exits.
Jean and Berenger enter at the same time, from stage right and stage left. Jean is very neatly dressed with polished shoes, a new shave, and tidy clothing. Berenger is the opposite: scruffy with wrinkled clothing, an untucked shirt, and messy hair.
They sit at a cafe. Jean immediately criticizes Berenger for being late for their rendezvous, so Berenger asks when Jean himself arrived. Jean acknowledges that he arrived at the same time as Berenger, but the difference is that he does not like to wait. Jean then confronts Berenger about how shabby he looks and how much he drinks. Berenger accepts the criticism. He explains how bored he gets in the town and how little he enjoys his work.
Just as Berenger begins to describe his previous night of drinking, a strange noise is heard, far off, of a beast panting. As their waitress takes their order, the noise becomes louder and louder. Soon the two men have to shout in order to continue their conversation.
Just as the noise escalates to its highest level, the characters on stage see a rhinoceros. A number of almost simultaneous actions follow. The Grocer's Wife calls her husband, The Grocer, who sees the beast. The Housewife with the basket of provisions drops her groceries—but not her cat. An Old Gentleman, elegantly dressed with a cane, helps her regroup. The Cafe Proprietor demands to know what is going on. The Logician enters wearing a straw hat and an eyeglass. No one can believe his eyes—it is a rhinoceros! As mysteriously as it arrived, the rhinoceros (with the noise) disappears.
Every character on stage—except Berenger—repeats the same line: "Well, of all things!" Each person tries to resume what he or she was doing before, and new conversations ensue. The Grocer convinces the Housewife to buy from his grocery store. The Old Gentleman helps the Housewife collect her fallen items. The Logician holds her cat. Meanwhile, Jean and Berenger order drinks from the waitress. Jean asks Berenger what he thought of the rhinoceros, and Berenger somehow seems less shocked than all those around him. Jean is shaken to the core, but Berenger yawns and responds, "[I]t won't reach us here."
The Logician explains to the Old Gentleman the definition of "syllogism": it "consists of a main proposition, a secondary one, and a conclusion." But this line of somewhat rational conversation ends when the two leave. Jean and Berenger get into a heated discussion. Because his friend does not respond with shock about the rhinoceros running around town, Jean accuses Berenger of dreaming. Berenger responds by saying he is not dreaming, but Jean says that dreaming asleep and dreaming awake are the same thing. Jean accuses Berenger of insulting him, and Berenger says he never intended to do so. They continue fretting about the rhinoceros. Jean orders Berenger not to drink his pastis, but Berenger does anyway.
Just then Daisy, the pretty typist who works with Jean and Berenger, passes by. When he sees her, Berenger stands up and spills Jean's drink on him. This angers Jean, but Berenger asks him not to yell so that he can hide from Daisy. Berenger does not want her to see him in this embarrassing position. After Daisy exits, Jean continues to press Berenger about why he drinks so heavily, and Berenger admits that he drinks so that he can feel more himself. He says the world is boring otherwise.
Jean accidentally knocks the Logician, who has returned, with his arm. At that, the Logician's conversation with the Old Gentleman about syllogisms resumes. The Logician suggests that since cats have four paws, anything with four paws is a cat. The Old Gentleman seems to accept this kind of logic.
Jean's and Berenger's dialog continues, and Berenger describes his particular state of mind. He wonders if he wants to go on living. He wonders if he really exists. Jean cannot understand these existential conundrums and blames Berenger's alcohol consumption. The Logician's and Old Gentleman's conversation becomes increasingly bizarre. With the Logician's "logic," they determine that Socrates was a cat. Berenger tells Jean that he needs to sharpen his mind and become a cultured man. The Logician and Old Gentleman discuss illogical math problems as if they make sense. Finally, Berenger promises Jean that he will stop drinking and invest in cultural activities and intellectual pursuits. But when he asks Jean to come to a museum with him that afternoon, Jean says he will be napping. When Berenger asks him to go to a play that evening, perhaps a play by Ionesco, Jean says he will be out drinking.
The galloping sound returns. As Jean explains why it is okay for him (but not for Berenger) to drink, the two men have to scream to be heard. Then, when the sound again reaches its climax, the characters on stage say, one by one, "oh, a rhinoceros!"
When the rhinoceros has departed, the housewife cries because the beast has run over her cat. As the Proprietor tries to comfort her with brandy, Berenger, Jean, and the Old Gentleman argue about the rhinoceros. Were there two separate animals, or was it the same one? What is the difference between an African rhinoceros and an Asian one? Was there enough time to notice the animal's physical details as it stormed by? The conversation is convoluted and confusing.
To settle the increasingly complex debate, everyone turns to the Logician. He speaks with confidence but makes no real sense. The others listen intently to him, however, and trust his authority. The Logician then exits, and all declare that they will not stand for this rhinoceros ravaging the town. As the act closes, Berenger scolds himself for arguing with Jean and decides to drown his sorrows by drinking more.
The title character in this play, the rhinoceros (or maybe there are two), appears briefly but affects everyone in the play. That the title character is a rhinoceros suggests from the start that the play is something of an allegory. Indeed, the characters and plot point to some kind of moral, which Ionesco teases out over the course of the play. To help the audience understand that a general message will be conveyed, Ionesco establishes an "anywhere" setting, a place in the center of the town where regular people convene, to represent any community. Stock characters such as the Grocer, the Waitress, and the Housewife represent common, generic figures. These people are never fleshed out. At best, they are allegorical characters, symbolizing different philosophical perspectives. In both his choice of setting and his choice of characters, Ionesco maintains the Absurdist tendency towards stylized allegory rather than realism.
Ionesco also quickly establishes that Berenger and Jean are foils. They arrive together from different places. Berenger, with his scruffy appearance and wrinkled clothes, clearly represents a kind of individualism in contrast to Jean, so clean and put together, more of a conformist. The philosophical distinction between these two figures will become clearer in scenes to come.
The first act also introduces a fundamental theme: valid reasoning versus false logic. The Logician would provide an important counterpoint to the absurd conversation between Jean and Berenger, but his reasoning is terribly deficient and misleading. He defines "syllogism" well enough, but he seems unable to distinguish the major premise from the minor premise when it comes to a real application of the definition. At the end of the act, as he determines how many rhinoceroses there were, the Logician maintains an air of authority, which seems to be good enough for those assembled. Although he makes no real sense, the Grocer, the Old Gentleman, and the other townspeople accept his reasoning because of his authoritative position. Logic and truth will take on greater urgency through the rest of the play. Unfortunately, the people are not rational enough to handle more than the simplest reasoning or even maintain more than the most superficial of conversations. The people cannot think for themselves and are content for the Logician to play the role of demagogue.
At the close of the act, the audience has an unsettling feeling of danger. Not only is a rhinoceros terrorizing the town, but the people are unable to come up with something to do about it. All they know is that rhinos generally should not be allowed to rampage.
Critics have noted that Ionesco was interested in this theme since his own experiences in early childhood, and the idea of futility resurfaces throughout his plays. As with Dadaism, an artistic movement which broke down ordinary sounds and actions until they were units of nonsense, Ionesco was not the only one to question the worth of everyday life by demonstrating the irrational side of human nature.
In this context, Berenger orders drinks to drown his sorrows. He does not seem to care to be socially or personally responsible. What does being responsible entail, anyway, given the apparent futility of ordinary life? Is it possible for Berenger to become happy? What must he do to regain hope? Where is the meaning in ordinary, everyday actions? And how much better, after all, are we in asking worthy questions and using our powers of reason to answer them? The close of the first act has made us question the extent of our ability to be rational actors in a confusing world.