Dr. Palmer Jekyll is a scientist who once lived in a mansion in Vermont with his daughter Marianna. His niece Heidi Davidson, who he and Marianna hadn't seen in seven years, comes to live with them after her parents die in a car accident. When Heidi first came to town, she was told by a boy named Aaron Freidus about a monster attacking the town on a nightly basis.
When she tells Palmer, he reacts angrily, trying to claim that it is all a story made up by the townspeople. However, soon Heidi begins to see more and more things that prove that there is a monster. She believes it to be Palmer, especially when he returns home one night, clothes torn and body a mess. But he claims to have injured himself on an evening jog.
Heidi eventually finds a diary that confirms her suspicions that Palmer is hiding the truth about a monster. When she sneaks into Palmer's lab and sees him drinking a green liquid that makes him react in a monstrous fashion, she believes her suspicions to be true and tries to leave. However, Heidi soon runs into the monster, who tries to attack her. Eventually, Heidi is saved by Palmer, who fights off the monster until it suddenly transforms into Marianna.
They then reveal to Heidi that five years ago, Palmer had car trouble which caused he and Marianna to wind up stuck by a forest. When Marianna went off on her own into the woods, she was bitten by a monster. The result caused Marianna to transform into a monster every night. That's why Palmer was testing formulas on himself, to find a cure for her. Suddenly, the townspeople arrive, intent on killing the Jekylls. The Jekylls start to make their escape, but Heidi returns to the mansion to grab the diary. When she makes it back to the tunnel, the Jekylls are long gone.
Palmer is described as being genuinely nice, particularly towards Heidi, despite being quick to anger when she first mentions the monster. This is mainly due to the entire situation surrounding Marianna. He is also fascinated by the story Dr. Jekyll and Mr. Hyde.
Palmer is described as having white hair, red rimmed eyes with craggy wrinkles and sickly pale skin.
- Despite the cover depicting him as a monster, Palmer never actually turns into a monster in the book.