The House of Dies Drear begins with a dream. Thomas Small is in a pine forest running from a scary man with a white beard. When he wakes up he realizes he's in a car with his parents and his toddler twin brothers, Billy and Buster. It's Saturday, and it's his thirteenth birthday. The Smalls are driving away from their old home in North Carolina to their new home in Ohio. Their new house is historic – it used to be a stop on the Underground Railroad, the system of people and places that helped slaves escape to freedom back in the pre-Civil War era. It has hidden rooms and secret passageways. Thomas's father is really excited because he's a historian and teaches US Civil War history.
Thomas loves history, too, and really wants to check out those hidden passageways. He's excited for another reason as well. It so happens the house might be haunted – by the man who used to owned the house, Dies Drear. Drear was an abolitionist, a person opposed to slavery. He gave runaway slaves food, shelter, money, and kindness. Drear was murdered by bounty hunters a hundred years before, sometime in the 1860s.
The house is also supposedly haunted by other ghosts. Thomas knows this because he read the report his father got before he agreed to rent the house. The report also talked about three slaves running for Canada after they left Drear house. Two of them were murdered. Mr. Small got angry when Thomas read the report. Thomas can tell his father is keeping something from him and is determined to find out what.
When they arrive at the new house, Thomas is extremely excited, and a little scared. He sets about exploring right away. After meeting some kids that live near him, Mac Darrow and a little girl Pesty, he finds himself trapped under the house, until his mother slides open the wall and lets him in to the kitchen! Then he does more exploring and has a run-in with what looks like a monster, possibly the caretaker of the place, a man who's called Mr. Pluto. Thomas is very tired at this point. He sleeps in the living room so he can protect the house from any intruders. While he's sleeping, someone, or something, slips into the house and then out again.
In the morning, Thomas wakes to find a new Sunday suit. He's happy and excited to be going to a new church this morning. His parents are excited for another reason. In the night, someone, or something, has been in the house. When they get to church, they are happy to be there, but find the townspeople unfriendly. Mr. Pluto is there with Pesty. Mac is there playing the piano. Mac's father and three brothers are also there, and they look mean. After church Thomas feels sad. He was hoping people from church would come over to their house to visit, like they did back in North Carolina.
Mr. Small takes the family to the college campus, where he'll be working. Thomas gets very upset in his father's office. He feels unwelcome in the new town. After a little lunch, though, he's back to his excited self. The Smalls spend the afternoon driving around town, and Thomas is really happy.
When they get home they find that someone, or something, has made a huge mess in their kitchen. Afraid Mr. Pluto is to blame, Thomas and his father go to Mr. Pluto's house, which happens to be in an underground cavern. When Mr. Pluto sees them he runs into his cavern home. Mr. Small and Thomas force their way in and find themselves in the middle of an amazing treasure! Mr. Pluto begins to explain, and soon, another Mr. Pluto comes into the room. Thomas gets really excited and tries to attack him. The man's hair and face come off in Thomas's hand.
Not really, though, he just thinks they do it first. What he's got on his hands isn't human hair and skin, but pieces of a mask and a wig. It's Mayhew Skinner, an actor, and the only son of Mr. Pluto (whose real name is Mr. Skinner). Mayhew and Mr. Pluto explain that the treasure belonged to Dies Drear, the owner of the house. Mr. Skinner found it long ago. The Darrows had also heard there was treasure, and had been scaring Mr. Pluto for years, trying to get him to leave the property so they could look for the loot.
They don't know Mr. Pluto has already found the treasure. They were the ones that snuck into the Small's house in the night and vandalized the Small's kitchen. They've been trying to scare them away like they tried to do with Mr. Pluto. Mayhew is in town because he's afraid his father is sick, and because he wants to make sure the Smalls are nice people. He also wants to stop the Darrows from harassing his father. Mayhew has been dressing up like his father so nobody will know he's in town.
Mayhew, Mr. Pluto, Pesty, and the Smalls hatch a plan to put on a "play" to scare the Darrows. The next day (Monday) they put the plan in action. They trick the Darrows into believing that Mr. Pluto is in the hospital, and his cave unguarded. Then they wait in the cold and the darkness for the Darrows to come. When they do, everybody plays a part in scaring them badly, pretending to be ghosts and monsters. Terrified, and the Darrows run away.
Mr. Small agrees not to tell anyone about the Dies Drear treasure until he's had a chance to catalog all of it. This means that Mr. Pluto can keep living the way he has been for years to come. The next morning Thomas wakes up, eats a quick breakfast, and charges over to Mr. Pluto's cave. His dad and Pesty are already there with Mr. Pluto, cataloguing the treasure. Thomas feels happy, and like he's had enough adventure to last him at least a little while.