Driskoll wants a pet, and is pestering the boys' father, Torin for one. But Torin is troubled by a strange sickness that is pervading the town, and doesn't have the time, energy or effort to devoting to Driskoll's request for a pet. Irritated by his son's constant bothering of him for permission to get a pet, Torin tells Driskoll to clean his office to prove to his father that he is mature enough to care for a pet.
Driskoll starts to clean half-heartedly, but begs for Kellach's help. Kellach freezes the floor of Torin's office with a new spell he's learned, but the slipping and sliding this engenders provides more of a hinderance than a help. Torin lambastes the two boys for turning his office into a playground and denies Driskoll a pet until he can grow up and be mature.
On the way home, the two stop into the Church of St. Cuthbert's to check out the strange sleeping sickness that Torin had been investigating. Much to their surprise, the temple is full of wolves and snakes, including a small, green snake that Driskoll saves from the wolves. To Driskoll, this little snake, that he names Suma, would make the perfect pet. Now, all he has to do is take care of it.
The two beg help of Moyra, who takes the boys to an animal healer so he can check Suma out. She takes them into the old part of Curston, to an animal healer named Selik. Selik says Suma is perfectly healthy and sends the boys home with several clay pots he claims will make a perfect home for the snake. But because Selik cheats Driskoll out of all his money in selling him the pots, Moyra takes a chance to even the score by stealing a book about snakes from Selik.
But during the night, the clay pots are broken, and the only one who could possibly have broken them was Suma. But how could a snake without arms break the clay pots? Overnight, more of the city falls prey to the sleeping sickness, including Zendric, Kellach's mentor. When Driskoll pays a visit to the blacksmith's who sharpened his sword for him, the forge is attacked by a half-man, half bull creature of metal. The creature seems fixated on the brothers, but they manage to give it the slip.
At Zendric's, Kellach discovers his mentor is clutching a slip of paper talking about a creature called an Inevitable, and how they are supposed to use it to do something. Kellach reads the book Moyra stole from Selik and discovers that the tattoo appearing on people's faces indicates that they were bitten by a snake-like creature called a Yuan-Ti. In fact, the design on the walls of Selik's rooms was made by the very same Yuan-Ti, whom Selik is working with for some nefarious purpose.
More and more people are succumbing to the sleeping sickness. Even Moyra and the boys are falling prey to it, but Kellach works out that Driskoll's new pet is himself a Yuan-Ti, and the snake admits it. The city is infested with the Yuan-Ti, who are obeying their ruler and stealing the dreams of the city's residents. When Driskoll saved Suma from the wolves at the temple, however, Suma threw in his lot with them, and is trying to save them from the fate that is befalling the rest of the town. If all the human's dreams are stolen, the humans will die. The leader wants the dreams to bring him the power and creativity of the humans.
But can three children and a single Yuan-Ti prevail over the leader of all the Yuan-Ti when the leader has an entire tribe on his side?
In enjoyed the book, although this seems to be the last one of the series. Unfortunately, it's not the best of the series, not by a long shot, but revisits many of the series tropes, from missing parents and mentor to the kids having to take on a much more powerful enemy by themselves.
Parents will like this series for the fact that the kids act like kids and don't have adult-level reasoning skills, but manage to triumph anyway, along with no objectionable content, either language-wise or content-wise.