The cover illustration shows a monster looking through a tent in the leafless trees.
Billy Harlan has just been sent off to Camp Nightmoon by his parents, who are explorers and scientists. While on the bus to camp, Billy encounters many characters, such as Mike, the fat child who is scared of everything; Colin, who wears sunglasses and a red bandanna around his long brown hair; Jay, the jockish child with wild curly red hair and Dawn and Dori, two girls from the Girl Camp fraction of Camp Nightmoon. The bus driver stops the vehicle in the middle of a desert on the way to the camp and orders everyone out of the bus. He unpacks everyone's belongings and drives off, abandoning the children in the desert. The children are suddenly surrounded by a group of creatures who prepare to slaughter the children, until a man shows up on another bus with a rifle and shoots it at the creatures. The creatures run away in fear.
The man who saved the children introduces himself as the leader of Camp Nightmoon, Uncle Al. Uncle Al invites the children onto his bus and drives approximately five minutes to the camp from where they were dropped off.
Mike and Jay and Colin all bunk together. While preparing their cabin, Mike finds a pair of poisonous snakes in his bed. Jay jokes around and play pushes Mike into the snakes. But Mike gets bitten on his hand, which then starts copiously bleeding. He runs off to find the nurse, while Billy devises a plan to rid the snakes by wrapping them up in a bed sheet and throwing them outside. This plan is heralded as brave by their bunk's counselor, Larry, who then laughs when he is told that Mike went to find the nurse because there is no nurse. Mike returns and as his hand is described as bleeding profusely onto the floor of the cabin, Larry tells him to just wash his hand and wrap a bandage around it. The boys go to eat dinner around a campfire, where Uncle Al tells them the rules of Camp Nightmoon. The campers have to write home every day to tell their parents what fun they're having. They are not allowed out in the forest or along the river that runs between the boy's and girl's camps. This is peppered with a warning by Uncle Al that the forest is dangerous and infested with Tree Bears.
Uncle Al also informs the campers that they are not allowed to ever enter the Forbidden Bunk. Jay decides after the fire that he wants to sneak out to see the Forbidden Bunk. Larry overhears him and tells him he probably should not, as the Forbidden Bunk is where the Sabre lurks. Then the boys hear hideous howling from the Forbidden Bunk. The next day, the bunkmates are all enlisted to play something called Scratchball. This entails one person throwing a ball as far as they can, then attempting to run all four bases of a baseball diamond before someone catches it. Mike sits this game of Scratchball out, as his hand is swollen. Larry plays his spot and after Colin makes a move he does not appreciate, Larry loses his temper and throws the ball directly at the back of Colin's head. Colin is knocked to the ground and Larry helps both Mike and Colin to the main cabin to see Uncle Al.
Billy and Jay go back to the bunk and work on writing their daily letters home. Larry enters with Colin, who is miraculously just a little sore. Billy asks where Mike is but Larry just shrugs and leaves. Later, Billy discovers all of Mike's belongings removed from the cabin, and no one will tell him where Mike went.
Later that night, Jay introduces Billy and Colin to some child named Roger when a plan is devised for the two to sneak into the Forbidden Bunk. Billy and Colin decline the invitation to sneak out and Jay and Roger leave for a short time. Then there are screams and Jay runs back and tells Billy that Roger was attacked and mauled by the Sabre. The three boys lock themselves in the cabin, until morning.
When they do leave, they do not see any traces of the previous night's atrocities. They find Larry and tell him what happened to Roger. Larry seems a little confused but agrees to talk to Uncle Al about the missing camper. Billy goes down with some campers to the river for a swim, where he is met by Larry, who tells him that Uncle Al searched the Forbidden Bunk but could not find any traces of foul play, and what's worse, Uncle Al has no record of there ever being a camper named Roger at Camp Nightmoon.
At the river, Billy is accosted by Dawn and Dori, who have swam over from the girl's camp and hid in the bushes for hours on the hopes that Billy would walk by. The girls tell Billy that campers are disappearing from their camp as well, and they plan on making a break for it very soon. The three agree to meet again and Billy takes off back towards the bunk. On the way, he spots a payphone by Uncle Al's offices. He plans to call home to his parents and pleads for them to come to the camp and rescue him and all his friends. Unfortunately, the telephone turns out to be a plastic decoy. Uncle Al walks out of the bunk and informs Billy that campers are not allowed to use the telephone. And also Billy's going on a canoe trip tomorrow.
When Billy makes it back to the bunk, he tells Jay and Colin about the girls. Then the three of them decide to write to their parents and tell them precisely what is happening and hope for a speedy rescue. Larry stomps into the cabin and tells Colin and Jay that they are going on a special two camper hike with a counselor named Frank. The two campers and the counselor all mysteriously disappear with no explanation. After trying and failing to get answers out of Larry, Billy stumbles into Uncle Al's office. There, he finds a burlap sack full of all the letters he and his fellow campers had been writing, which have clearly never been mailed at all.
When Billy returns to his bunk after dinner, he meets his two new bunkmates, Tommy and Chris, who will also be joining Billy on his canoe trip tomorrow. Tommy and Chris also tell Billy that Uncle Al announced that Visitor's Day has been canceled, so no one's parents will be arriving anytime soon.
The next morning, Larry takes Billy, Chris, and Tommy out to the canoe and the four head off downstream. Larry decides he would like to look at the fish in the river. Unfortunately, he leans too far out of the canoe and falls in.
Billy bravely abandons his two bunkmates in the canoe and leaves them to float downstream, while he rescues the drowning counselor. Billy pulls Larry to safety and the two walk up the bank of the river back to camp. Larry tells Uncle Al that Billy saved his life. Billy keeps trying to get either Uncle Al or Larry to acknowledge that the two bunkmates were in the canoe, but Uncle Al finally pays attention and scolds Larry on losing his favorite canoe.
The next day, Billy is awoken early by Larry. Apparently, Uncle Al has called a surprise morning hike and the whole camp is required to attend. Billy and what's left of his campmates are walking in the forest when Uncle Al instructs his counselors to remove rifles from their bags and pass them out to the children. Two girls have escaped from the girl's camp, Uncle Al informs the campers, and Billy is excited that Dawn and Dori made it out. Uncle Al then instructs the children that they are to find the two girls, who are presumed to be hiding in the forest, and shoot them. Billy refuses this order and instead turns his gun on Uncle Al and fires.
The gun does not go off and Uncle Al gets very excited and declares "You've passed the test!" Dawn and Dori appear from within the forest. So does Jay and Colin and everyone else Billy had met at the camp. Then his parents show up and give him a big congratulatory hug. Billy is told that his parents wanted him to go with them on their next big exploration. But first he needed to be tested by the government to see if he was ready. Uncle Al tells Billy that he passed the test by Obeying Orders (not going into the Forbidden Cabin), Showing Bravery (rescuing Larry from drowning), and Knowing When Not To Follow Orders (refusing to shoot the girls). Billy is told about the strange place they're going to be exploring. A planet called Earth.
Cover Art Gallery
- Although the 9th book in the series, it is the first release to establish a monthly release schedule. Following this book, the original series would have 1 new book released every month until the end of the series.
- This book is similar to the television episode from The Twilight Zone television series, Third from the Sun.
- The UK version covers featured two campers being swept down the river.