The cover illustration depicts Heidi Davidson looking at a humanoid creature in her uncle's lab. Chemicals and glass can be seen on the ground.
Heidi Davidson is riding a bus heading to Shepard Falls in Vermont. After her parents died in a car accident, Heidi has been sent to live with her uncle, a scientist named Dr. Palmer Jekyll. She hadn't seen or spoken to Uncle Jekyll or his daughter Marianna in seven years, since she was five years old. Heidi is left at a bus station with nobody waiting for her. She is soon greeted by a boy her age named Aaron Freidus. When she tells him about her uncle, he panics. He tells Heidi that the town has had to deal with attacks by a horrible beast every night and that Dr. Jekyll is the one to blame. Heidi laughs at Aaron, believing he was just trying to scare her with the story of Dr. Jekyll and Mr. Hyde.
Heidi walks to Uncle Jekyll's mansion by herself. She opens the door, only to be stopped by Marianna, who acts cold and angry towards her, telling her to leave. Heidi instead heads to Uncle Jekyll's lab and greets her uncle, who is happy to see her. But when she tells him about Aaron's story, his face gets red. He yells at her that the monster stories are a lie. However, more and more things begin to sway Heidi's suspicions that Uncle Jekyll may be a monster. This includes finding a room with the wallpaper completely shredded. But Marianna tells her that this was the doing of their cat George, who was recently put to sleep.
Later that night, at dinner, Uncle Jekyll talks about the story of Dr. Jekyll and Mr. Hyde and how Jekyll was misunderstood. This angers Marianna until he decides to drop the subject. As Heidi sleeps, she hears screams outside. She goes into Uncle Jekyll's lab, he's nowhere to be seen, but what appears to be an empty cup that had once contained some sort of green liquid. She runs to the kitchen just as Uncle Jekyll arrives, his clothes torn up and his body covered in dirt. But he claims that he fell during a night jog. The next day, she tries to get answers from Marianna, but Marianna angrily lashes out at her, accidentally squeezing her hamster to death in the process.
Heidi leaves the mansion and heads into town, where she sees all the damage caused by the beast. She also sees Aaron being interviewed by a reporter. When she mentions that she's related to Dr. Palmer Jekyll, the townspeople advance on her, wanting the truth and wanting to go to the mansion. She returns to the mansion and later falls asleep. After having a nightmare about being attacked by a beast, she decides to look through the mansion library for any information on the beast. She comes across a secret diary that mentions that Uncle Jekyll does know about some sort of monster. She sneaks into Uncle Jekyll's lab as he drinks some kind of formula that makes him react in a monstrous manner. She tries to run, but he catches her and locks her in her room. He then leaves as another attack on the town commences.
Heidi escapes her room through the window and heads into town just as a giant explosion goes off. She sees the beast, who fixes its gaze on her and attacks, only for Aaron to show up and fight it off with a baseball bat. She returns to Uncle Jekyll's lab, just as the beast catches up to her. Suddenly, Uncle Jekyll shows up and saves Heidi from the beast, who transforms back into Marianna. Uncle Jekyll then reveals that he beast was Marianna all along. When Marianna was five, she had run off on her own into the forest while Uncle Jekyll was fixing their car. She was then bitten by some sort of beast, which cursed her with transforming into a beast every night. When she had seen Uncle Jekyll drinking the formula, he was testing it on himself to see if he could find the cure.
Suddenly, the villagers show up and start to tear the house apart, looking to finally kill the beast. Uncle Jekyll leads Marianna and Heidi down into the basement, where there is a secret tunnel leading away from town. The three get halfway down, when Heidi remembers an old diary she has found in her bedroom. Believing the diary to be important, she runs to get it, only to be attacked by the villagers, who start to burn the building down. Heidi is saved by Aaron and the two head into the tunnel. However, Uncle Jekyll and Marianna have long since vanished. As they exit the tunnel, Heidi takes one last look at the mansion and sees it in flames.
Later that night, while staying at Aaron's house, Heidi continues to read the diary and discovers some passages written by Marianna. This includes a passage mentioning that she had bitten Heidi while she was asleep, passing the curse on to her.
- In France, this book is the 57th of the original Goosebumps series. Its title was translated into "Le Manoir de la Terreur", which means The Mansion of Terror, and its tagline was changed to "Panique au village" (Panic in the village).
- The title is a spoof of the characters Jekyll and Hyde. Additionally, the book references Godzilla, Frankestein, and Doc Martens.
- Aaron Freidus, Heidi's friend, shares a full name with the protagonist of The Werewolf in the Living Room.
- Illustrator Tim Jacobus said that the cover for this book was one of the best in the series.
- The creature on the cover of this book was not the creature that appears in the story. There is a scene in the book where Uncle Jekyll drinks a formula that makes his body react in a monstrous fashion, but not to the point he actually transforms into a monster.
- The vaguely described "forest creature" encountered by Marianna could have been a werewolf, as it seems to possess some of the traits a werewolf might.
|Goosebumps Series 2000 series|
|#01 Cry of the Cat||#10 Headless Halloween||#18 Horrors of the Black Ring|
|#02 Bride of the Living Dummy||#11 Attack of the Graveyard Ghouls||#19 Return to Ghost Camp|
|#03 Creature Teacher||#12 Brain Juice||#20 Be Afraid — Be Very Afraid!|
|#04 Invasion of the Body Squeezers: Part 1||#13 Return to HorrorLand||#21 The Haunted Car|
|#05 Invasion of the Body Squeezers: Part 2||#14 Jekyll and Heidi||#22 Full Moon Fever|
|#06 I Am Your Evil Twin||#15 Scream School||#23 Slappy's Nightmare|
|#07 Revenge R Us||#16 The Mummy Walks||#24 Earth Geeks Must Go!|
|#08 Fright Camp||#17 The Werewolf in the Living Room||#25 Ghost in the Mirror|
|#09 Are You Terrified Yet?|