Two step-siblings, Gretchen Hughes and Clark Boretski, are being driven through the swamplands of southern Georgia to stay with their grandparents for a few days, while their parents attend to vague activities in Atlanta. Along for the ride is their dog Charley and also a flat tire, which has halted the journey as the book begins.
While waiting in the swamp for the family car to get fixed, Gretchen complains about the humidity and openly wishes she were spending the summer in Atlanta instead. She coerces her brother into accompanying her in a mild exploration of the swamp, navigating through tall reeds and grass. They spot something called a "Cyprus Knee" in the marsh, along with... a horrible, towering swamp creature with red eyes! Except, it is actually Charley the dog. The children also hear mysterious low moans arising from somewhere within the swamp.
It is revealed that Gretchen and Clark's grandparents live in a castle in the swamp. The castle has only three windows, one on each of its three stories. The grandparents greet the family, as they finally arrive. The elderly couple reveals that their castle has no television, no telephone, and no way out either since their car is being repaired. But luckily, there are plenty of chicken pot pies in the castle and the entire family sits down for a meal. Charley the dog starts barking at the floor.
The parents leave and Gretchen settles into her new bedroom, which is the size of a gymnasium and is adorned only with a bed, a dresser, and a lamp. After an uneasy night filled with more mysterious howls from the swamp, Gretchen goes next door to collect Clark from his bedroom and the two make their way down to the kitchen for breakfast. Grandma Rose is making blueberry pancakes. She hugs the two children, staining their clothes. Grandma makes enough breakfast to feed several armies, and Gretchen tells Clark that they have to eat as much of the food as they can to avoid insulting her. Grandma heaps ten pancakes on Clark's plate without eating a single one herself.
Gretchen remembers that her parents had given her mystery books to give to her grandparents and races off to her bedroom to retrieve them. En route to her bedroom, she spies Grandpa Eddie carrying a huge stack of pancakes down the hall. Gretchen decides she is going solve the mystery of why Grandpa Eddie was carrying pancakes later.
After breakfast, Gretchen and Clark get shooed out of the kitchen by Grandma Rose, who tells them to go play out in the swamp. They run into Grandpa Eddie, who is brandishing a large saw. He invites them to help him build a shed, but they decline. Grandma Rose invites the children to assist her in making her "sweet-as-sugar rhubarb pie," and even though Clark insists that they would be no good at making pies, the children find themselves mixing enough ingredients for three pies.
The children are again encouraged to explore, this time inside the castle. But they are warned by Grandpa Eddie to stay out of the far room on the third floor, as it is a forbidden supply closet. The children investigate their grandparents bedroom, finding the room filled with boxes of old newspapers and magazines. Inside their closet, there are numerous broken antique toys, such as a dusty rocking horse and an eyeless porcelain doll.
Clark decided to play Hide and Seek with Gretchen, calling out "Not It' as he runs away to hide. Gretchen scours the castle in search of him, encountering many empty rooms and an abandoned piano, finally deciding to cheat and hide herself, forcing Clark to be it. She spots a dumbwaiter next to the locked forbidden room. The forbidden room has a tarnished silver key sticking out of the lock and Gretchen resists temptation, until she hears a commotion from behind the door. Thinking her brother has cheated and is hiding in the forbidden room; Gretchen opens the door and spots a green-furred creature with a gorilla body and an alligator head.
The monster is concentrating on eating pancakes and doesn't notice Gretchen in the room. She turns to leave and he spies her. Dropping the pancakes on the ground, he trounces after her. Gretchen narrowly escapes the locked room, only to encounter Clark in the hall. Clark thinks she is bluffing and enters the forbidden room. A moment later he screams and the monster shrieks. Clark hoofs it out of the room, leaving the door open behind him. The monster escapes out into the hallway in pursuit of the children. The children run away in fear, dragging their dog Charley with them, who keeps trying to turn around and confront the monster. They make it down to the second floor and lock Charley in a bathroom to keep him safe. The children attempt to get their grandparents' attention, but can't seem to find them in the castle. As the two children hear the monster tromping about on the floor above them, they also hear the sound of a car from below. They look out the sole window to see Grandma and Grandpa in their newly-fixed car-- backing out of the driveway. The children run down to the bottom floor to get their attention, but can't open the front door. They are locked inside the castle with the monster.
The children run around the bottom floor, trying to find a way out. The kitchen window is nailed shut--from the outside. They hear the monster above them; he has made his way down to the second floor. They hear the sound of the piano's keys above, then a giant smash. Then another key-littered smash. The monster is picking up the piano and throwing it across the room upstairs, over and over. Keys and crashing, over and over from above, shaking the entire castle. All goes quiet for a moment, then the children hear the sound of their dog whimpering from above. Grabbing a heavy candlestick, Gretchen leads her brother upstairs to rescue their dog. Once they make it to the second floor though, the monster is nowhere to be seen and Charley is still safely locked inside the bathroom. They hear the car below again and lock Charley back in the bathroom. After racing downstairs, they see a pink slip of paper that has been slid under the front door. It is a telephone message from their parents, informing the children that they are going to be staying at their grandparents a little longer than they had anticipated. The car sound was from a worker from the General Store, who takes messages on his telephone for the elderly couple. The car has already driven away when Gretchen hears her brother calling her from the kitchen.
Gretchen asks where the monster is and Clark replies that he is still upstairs. He then points to the refrigerator. There are two sealed envelopes held by magnets on the door. Clark opens the first one and reads the letter aloud. The gist of the letter goes something like this: "Dear children, sorry we left but a swamp monster invaded our house and we've been afraid to leave. Now that you're here, we can leave. We went to get help and locked you in so you would not get lost in the swamp. You'll be safe so long as you do not let the monster out. If you let the monster out, you'll have to kill it." Clark is about to open the second envelope when the swamp monster appears in the doorway to the kitchen. The children flee and discuss a plan of action. Clark thinks the two should simply hide and wait for their grandparents to return, however Gretchen is pretty sure they are not coming back at all, and so she proposes that they must do as the letter instructed: they must kill the monster. The children run up the stairs to the second floor and once they are at the end of the hall, the monster cockily appears at the top of the stairs, brandishing a wooden stool. The monster looks at the children and then breaks the stool over his knee.
Gretchen has a plan. The stairwell to the third floor is missing some planks, as the children run up the stairs and then jump onto the banister, the monster will keep running and fall three stories down to the ground floor of the castle. The children jump up and grab onto the banister and watch as the monster indeed falls to his "death." The children make their way down to safety and Gretchen goes to the bathroom to retrieve the candlestick, thinking she will use it to smash the window open and make her escape. However, she sees that Charley is no longer in the bathroom and hears the unmistakable sound of the monster's groans. She races down to the kitchen where Clark is feeding Charley and informs her brother that the monster is still alive. Gretchen stuffs their dog into a small closet-like room and spies the rhubarb pie Grandma Rose baked. Figuring the monster has not eaten yet, Gretchen comes up with how to kill the monster: they will poison the rhubarb pie. Gretchen pours drain cleaner, rat poison, ammonia, and turpentine into the pie. As the monster makes its way to the kitchen, Gretchen throws some orange paint and mothballs into the pie for good measure.
The two children hide under the kitchen table as the large monster makes his way around the kitchen, sniffing at appliances. The creature rips the oven door off and throws it across the kitchen. He retrieves the non-poisoned pies from the oven and shoves them whole into his mouth, then turns around and polishes off the third, adulterated pie. The two children huddle in fear as the monster stands inches from them, licking the pie plate. Then the creature's eyes pop out of its head and it falls to the floor, dead. Gretchen does not want to get too close but it looks like the monster isn't breathing. Clark defiantly throws his mud monsters comic book at the monster's still head. The children go to retrieve their dog from the closet and discover the room he was in is actually a passageway to the outside! The siblings try to open the door leading out to the swamp, but like everything else in the castle, it is locked.
The monster appears, alive, in the kitchen doorway. Clark thinks it's going to eat him first since he threw the comic book at its head. Gretchen rightly reasons that the monster will likely eat both of them, since they did try to kill it and all. Gretchen, holding the shovel in both hands, attacks the monster in the stomach with the tool. The monster picks the shovel from her hands and tosses it behind him back into the kitchen. Gretchen tries to grab the other shovel from the wall of the room, but the monster spots her movement and grabs that shovel as well, breaking it in two with his hands. Gretchen tells her brother to open that second envelope from the fridge. Before Clark can comply, the monster grabs Gretchen's arm and pulls her closer to him, her face now in front of the creature's rank, gator-like mouth. Charley the dog runs to the rescue, biting into the monster's leg. The monster kicks his leg backwards, sending the dog flying across the room back into the kitchen.
The monster pulls Gretchen's head into his mouth and extends his tongue, covering her entire face with the creature's saliva as he licks her body to prepare for eating her. Before the monster can close his jaws around Gretchen's head, he stops, removes her from his mouth and asks, in English, "You human?" Gretchen confirms she is. The monster then bellows, "I'm allergic to humans!" The monster dies, for real this time.
The children do not stick around this time to make sure the creature is really dead. They get their dog and start walking through the swamp. After they have walked some distance, they notice the sky has grown darker. Gretchen suddenly remembers the second envelope and makes Clark open it. Inside is a second letter, warning the children that if they do kill the monster, they should not try to escape through the swamp, as the monster had many brothers and sisters. The children's grandparents had seen the other creatures at night, whistling to each other, and knew they were angry at the humans for capturing their brother. The grandparents warn their children to stay inside the safety of the castle or only travel to town using the main road. After they have finished the letter, the two children stop and listen to the sound of whistling from all areas of the surrounding swamp. And Clark asks, "Got any more ideas?" And Gretchen says, "No Clark, I don't, do you?"
- In the television episode, their dog, Charley does not appear.
- In the television episode, Gretchen and Clark put the ingredients into a gumbo, not a pie. They also grab more ingredients from the cupboards.
- In the television episode, the monster does not speak at all. Gretchen figures out that the monster is allergic to humans, when Clark accidentally sneezes on the monster (because of allergies).
- The first letter did not mention to kill the monster. The second letter did not mention that the monster had brothers and sisters; it rather generally stating that there are more of its kind. It also does not state not to go through the swamp; rather, warning not to be near the swamp at night.
- This book had a very similar beginning to Monster Blood as Gretchen and Clark didn't want to stay with their grandparents while their parents went to Atlanta and they encountered a creature later on in their stay. This was also what happened to Evan Ross when he was staying with his Aunt Kathryn.
- The Swamp Monster will make an appearance in the 2015 Goosebumps movie. He is one of Slappy's monster and villain henchmen and is referred to as the Bog Monster. His appearance, however, is different: while in the book he is described as a furry creature with an alligator head and a gorilla body, in the movie he looks like a giant made of moss.
Television Episode Trivia
- This is the third of four television episodes on the Monster Blood DVD.