After being taken away from their horrible guardian Count Olaf, (who tried to steal their fortune), the three Baudelaire children are taken by Mr. Poe to their new guardian, Dr. Montgomery Montgomery, who lives on Lousy Lane, which smells like horseradish. According to Mr. Poe, Dr. Montgomery is the Baudelaire’s “late father’s cousin’s wife’s brother.”
Dr. Montgomery, or “Uncle Monty” as he prefers to be called, is a short, chubby man with a round red face. He invites the children in for coconut cream cake. He is much friendlier than Count Olaf, and gives the children free rein in the house. Each of the children can have their own room. Monty tells the children that they will be going on an expedition to Peru, once his new assistant, Stephano, arrives. He says that his old assistant, Gustav, had suddenly resigned (Gustav could possibly be Gustav Sebald).
The children are fascinated by the many snakes in the Reptile Room, a giant hall in which Monty’s reptile collection is stored.
They meet The Incredibly Deadly Viper, which Monty has only recently discovered. The snake’s name is a misnomer since it is harmless; Monty intends to use it to play a practical joke on the Herpetologist Society in revenge for their ridiculing his name, Montgomery Montgomery. The three children are each given jobs in the Reptile Room: Violet is given the job of inventing traps for new snakes found in Peru, Klaus is told to read books on snakes to help advise Uncle Monty and Sunny’s job is to bite ropes into usable pieces. She also befriends the Incredibly Deadly Viper.
When Stephano arrives, the children realize that he is Count Olaf in disguise. They try to warn Monty, but Stephano foils these attempts. Eventually, Monty does realize Stephano is evil, but believes Stephano to be an impostor sent to steal the Incredibly Deadly Viper. Monty explains this all to the astonished orphans and tears Stephano’s ticket to Peru up, saying that Stephano will not be going on the trip with them. Stephano threatens the children privately later, hinting at a plot he has for them when they reach Peru. They tell him that Monty won’t let him go with them and Stephano becomes furious. On the day they are to leave for Peru, they discover Monty’s dead body in the Reptile Room. He has two tiny puncture holes under his left eye, and Stephano claims that he has been bitten by a snake.
Stephano still intends to take the children to Peru, where he will more easily find a way to get his hands on their fortune. However, as they are leaving the estate, Stephano’s car crashes into that of Mr. Poe. They return to the house, where Poe and Stephano discuss what to do with the children. The Baudelaires try to prove that it was Stephano who killed Monty.
Meanwhile, the children realize that they’ll need evidence to expose Stephano’s scheme. Klaus and Sunny stage a diversion in which the Incredibly Deadly Viper pretends to attack Sunny to allow Violet time to find and open Stephano’s suitcase. Stephano blows his cover when he informs Mr. Poe that Sunny is not in danger as the viper is harmless; he had previously claimed he knew nothing about snakes. Violet shows up and presents Mr. Poe with the evidence (among other things, a powder puff and the syringe used to inject snake venom into Monty). Mr. Poe asks Stephano to display his ankle, where the tattoo of an eye should be. However, the eye is not there. The Baudelaires insist that he has covered it with makeup. Mr. Poe wipes the ankle with a handkerchief, revealing the eye. However, Olaf gets away before he is arrested.