The people in the little rural town of Bjerget are eagerly awaiting the return of Rasmus, the younger son of Jeppe and Maren Berg, who has been away at the University in Copenhagen and are now coming back to visit his family and his fiancee Lisbed, the daughter of the wealthy Jeronimus.
Rasmus turns up, very proud of his newfound learning and eager to show off. He insist on being called by his latinized name Erasmus Montanus, peppers his speech with Insane Troll Logic and Gratuitous Latin and falls into disputes with just about everybody he meets. At first the townspeople are impressed and a bit confused, but when Erasmus starts to insist that the Earth is round, confusion turns to horror and anger, and they denounce him, lead by Jeronimus, who will not have his daughter marry such a madman. Both his parents, his brother and Lisbed tries to make Erasmus change his mind, but he insist that he alone knows best.
In the end, the townspeople enlist the help of The Lieutenant, who tricks Erasmus into joining the army. After a few hours of Training from Hell, Erasmus is ready to renounce all his ways and swear that "the earth is as flat as a pancake!" The humbled Erasmus is readmitted into the community and allowed to marry Lisbed.
This play provides examples of:
- Absent-Minded Professor: Erasmus after his university years. Jacob runs with it and laughs his ass off, telling how Erasmus managed to fit the trope a number of times: Almost falling off his carriage because of his antics, completely forgetting to put on his raincoat (which Jacob wisely used for himself), ignoring a dog that nearly bit him, and to top it all, walking straight into the barn, lecturing the cows. No wonder Jacob had the time of his life.
- The Alcoholic: Deacon-Peer has shades of this:"I only drink schnapps to help my digestion ...but I have a very weak stomach."
- Author Avatar: The Lieutenant's final speech about what you ought to learn in school is very clearly Holberg's own thoughts on the subject.
- Book Dumb: Deacon-Peer obviously. Also Hieronimus, the father of Lisbeth. The point is proven when the debate on whether the earth is round or not escalates.
- Deus ex Machina: The Lieutenant, who resolves the whole plot, are not introduced until Act 5.
- Drill Sergeant Nasty: Niels Corporal, giving Erasmus two hours of Training from Hell.
- Gratuitous Latin: Lots of it. Serves a a Genius Bonus, because a small knowledge of Latin makes the play even funnier.
- Gray-and-Gray Morality: Concerning the solution, because Erasmus has to withdraw his statement that the Earth revolves around the sun. We, and supposedly Holberg himself, know this is correct, but the peasants in the play — denies it. Erasmus might, however, have gotten his point through if he hadn`t been so blatantly arrogant.
- Honor Before Reason: Erasmus refuses to admit that the earth is flat, despite being ostracized and his engagement broken of, because he knows that it's round. Jacob deconstructs this, pointing out that the peasants won't believe Erasmus no matter what he does, so he might as well placate them.
- Insane Troll Logic: This is very much Erasmus' schtick, to the point where the concept of Insane Troll Logic is known as Erasmus Montanus-logic in Denmark. The page quote is the most quoted example, but he also "proves" that Deacon-Peer is a rooster, and that excessive drinking is the road to happiness.
- Insufferable Genius: Erasmus again. As with Insane Troll Logic, an Insufferable Genius is known as an Erasmus Montanus in Denmark.
- Know-Nothing Know-It-All: Deacon-Peer is very proud of his education, but for someone with just a passing knowledge of latin, or just basic knowledge of what latin is, he has clearly been very poorly educated, or forgotten about everything. Among other things, he claims several latin words have their origin in danish and that the language may have changed since he studied. Being a dead language, it obviously hasn't.
- Magic Versus Science: Closely verging on Clarke's Third Law, because Erasmus appears to know things that are so far out of the normal that he appears out of his wits, and is accused of having struck a Deal with the Devil when he tries to explain why the earth is round and revolves around the sun. When he uses his syllogisms to prove that people are animals, it is perceived as true magic, and his mother really believes her son has turned her into a stone. Because of this, even the officials begin to fear Erasmus, and this, more than anything else, produces the deal with the lieutenant.
- Mama's Baby, Papa's Maybe: Invoked against Hieronimus by his wife. It puts him out of balance, and when he asks his daughter for support, she only confirms what her mother just said...
- Obfuscating Stupidity: Erasmus' brother Jacob plays up his stupid peasant-act, while clearly being a lot cleverer than his brother. Whenever they debate, it is always Jacob who comes out on top.
- Only Sane Man: Jacob again. He is the only one who realize from the beginning how silly Erasmus is acting. Later, the lieutenant also takes on this trope.
- Plot Hole: It is established in the beginning of the play that Deacon-Peer is Erasmus' old teacher and a friend of his parents. However, when Erasmus meets him, he does not seem to know who he is.
- Pragmatic Hero: Jacob is a proponent of this, as he explains to his brother. Having principles (in this case, knowing the Earth to be round) is well and good, but when faced with someone who refuses to accept those principles and will make you suffer for holding to them, it's better to just go with the flow.
- Surprisingly Realistic Outcome: Erasmus' "debate" with the Deacon results in this, since the peasants observing have no idea what either of them are talking about. Deacon Peer is just rambling nonsense, but the better-educated Erasmus has no logical way to respond to any of it because it is complete nonsense. From an outside perspective, however, it looks like Peer put Erasmus in his place, and Erasmus is just trying to save face.
- Wham Line:"No, I can turn you into an actual soldier. You see, I am a recruitment officer, and you just took my coin. Therefore, ..."
- Working-Class People Are Morons: Zig-zagged. From the onset, they clearly are, not understanding the clever Latin Erasmus uses, and completely disdaining the modern heliocentric paradigm. On the other hand, as Jacob states, "If we peasants were to abandon our crops and just look at stars all the time, you learned men soon would experience empty bellies..."