I read it so long ago, I can't remember much at all so here's the plot from Wikipedia I hope it helps--
The second tale of the orphans begins with the children bumping down Lousy Lane in Mr. Poe's cramped car, headed to the home of their new guardian, Dr. Montgomery Montgomery, a herpetologist. 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 is much friendlier than their previous guardian, Count Olaf, giving the children free rein of the house. 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).
Monty treats the children well, and they 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 actually completely harmless; Monty intends to use it to play a practical joke on the Herpetologist Society in revenge for their ridiculing his name, Montgomery Montgomery. In the 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. Sunny's job is to bite ropes into usable pieces. She also plays with the Incredibly Deadly Viper, who becomes a friend of hers.
When Stephano arrives, the children quickly realize that he is actually Count Olaf in disguise. They wish to tell Monty, but Olaf threatens them with a knife. He is able to foil their attempts to warn Monty, but eventually their guardian realizes that something is wrong. He knows that Stephano is evil, but seems under the impression that he is an impostor sent to steal the Incredibly Deadly Viper. He 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. Olaf threatens the children privately later, hinting at some plot he has for them when they reach Peru. They tell him that Monty won't let him go with them and Olaf becomes furious. On the day they are to leave for Peru, they discover Monty's corpse in the Reptile Room. He has two tiny puncture holes under his eye, and Olaf claims that he has been bitten by a snake.
Olaf 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, Olaf's car crashes into that of Mr. Poe. They return to the house, and Poe and Olaf discuss what to do with the children. The Baudelaires try to prove that it was Olaf who killed Monty. Dr. O. Lucafont (an anagram of Count Olaf), arrives and claims that it was the Mamba du Mal that killed Monty (ignoring the orphans' repeated claims that the Mamba du Mal was locked up all night and was still locked up).
Meanwhile, the children realize that they'll need evidence to expose Olaf's scheme. Klaus and Sunny stage a diversion where Sunny goes to the Incredibly Deadly Viper, and screams at the top of her lungs as it wraps itself around her and Klaus shouts for help. While Mr. Poe panics, Violet finds Olaf's suitcase and picks the lock. While this is going on, Olaf informs Mr. Poe that Sunny is not in danger, since the Viper is harmless. He quickly realizes that he has blown his cover by demonstrating this knowledge of snakes (he had previously insisted that he knew nothing about snakes to prove his innocence). 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 Olaf 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. Olaf flees the house as it is discovered that Dr. Lucafont is actually Olaf's henchman, the Hook-Handed Man.
The novel concludes with Uncle Monty's reptile collection being taken away by a man named Bruce, and the Baudelaires being sent to their next home.