Jason Sloves is always bring teased and pranked by his older sister, Claudia. After Claudia teases Jason for having a babyish bedroom, his parent provide him with two new pieces of antique furniture: a large mirror and a dresser. Jason finds a note lying before the furniture, and it warns him that he's brought evil into his house. Jason assumes this is one of his sisters tricks.
Strange things begin happening with the mirror. After Jason's dog, Buzzy, is left alone in front of the mirror, he attacks Jake. After Jason's friend, Fred, stands in front of it, he seemingly becomes far more aggressive and competitive. Jason also starts seeing ghostly shapes in the mirror. Some of them look like Fred and Buzzy.
The mirror is actually the portal to a strange dimension inhabited by crab-clawed monsters. These creatures shapeshift and assume the identity of humans and animals from the real world. Jason ultimately has to enter the mirror-world, and he finds Fred and Buzzy. The group only escapes because Jason's pocket mirror can be used to burn a hole through the barrier that leads to the real world.
Jason destroys the mirror. While he cleans up the glass, a monstrous snake emerges from the dresser. It asks if its cautionary letter was received.
Near the start of the book, Jason contemplates whether or not he is a wimp; he ultimately concludes that he's not exceptionally wimpy. However, his sister's tricks get to him. Jason does eventually prove he can be courageous by saving Fred and Buzzy from the mirror.
As stated early in the book, Jason is very concerned with his appearance. He's always looking in mirrors, and he carries a pocket mirror.
Jason has thick black hair. Additionally, despite being twelve-years-old, he has a round, babyish face.