"The Slippery Slope" is the tenth installment of the "Series of Unfortunate Events" series of books. It relates another episode in the adventure of the Baudelaire children, Violet, Klaus, and Sunny, in their attemps to escape the evil Count Olaf, who wishes to steal the fortune left to the children after the death of their parents in a house fire.

The book opens as Sunny, the youngest of the three, is being taken up into the Mortmain Mountains by Count Olaf and his troupe of followers while Violet and Klaus are hurtling down the mountain in a runaway caravan wagon. The older children manage to stop the wagon and begin to make their way back up the mountain to find their sister. With the help of a boy named Quigley Quagmire, whom they meet on the way, they discover the ruined headquarters of a mysterious organization called the VFD where they decipher clues that may point to the possibility of their parents being alive. Quigley is also in search of family, having been separated from his siblings after it was thought he himself died in a fire. As the three children share their experiences they learn they have followed a similar path leading them to the VFD headquarters.

The VFD, the children determine, is a secret society to which their parents and Count Olaf belong, but which underwent a split with Olaf and his followers seeking to destroy the other members. As they learn more about the VFD it becomes clear that seemingly unimportant bits of information they were

made to learn as young children have significance in deciphering VFD codes.

While Klaus and Violet work on a plan for her rescue, Sunny, who is forced to act as a servant to Count Olaf and his troupe, learns what she can about his plan to thwart the VFD by destroying their last safe location at the Hotel Denouement. She also learns that Olaf holds a group of documents called the Snicket File, which was saved from the ruins of the burned down VFD headquarters and which may hold the explanation of where the Baudelaire's parents are.

Violet, Klaus and Quigley climb a frozen waterfall to the summit of the mountain where Count Olaf is camped and rescue Sunny, escaping by sliding quickly back down the icy slope. As they reach the bottom of the slope, the ice thaws and cracks, sending the Baudelaire children down one branch of a swift river and Quigley down another. The book ends on this note, setting the scene for the next installment in a subsequent book.