The Mirror Monsters live in a mirror dimension, and can only access to our dimension through a mirror. The story begins when the mirror is gifted to Jason Sloves with a wooden dresser. During the dinner of the Sloves family, one of the Monsters replaces Buzzy, Jason's dog. The Mirror dog attacks Jason and bites him in the neck before Mr. Sloves, Jason's dad, locks him in the garage.
Later that day, Jason invites his friend Fred to play video games. They see the real Buzzy in the mirror and call Jason's sister, Claudia, but the dog disappears before she can see it. Jason goes downstairs and one of the Mirror Monsters takes the opportunity to replace Fred, before leaving.
The next day, at school, Jason meets Fred's doppelganger in the gym locker room, but the Monster doesn't answer Jason's questions. The two of them go to play soccer, and the Mirror Fred plays aggressively, unlike the clumsy real Fred. After the game, Jason jokes by saying Fred seemed possessed. The Monster impersonating Fred reacts aggressively and starts punching Jason, which results in him being suspended.
Jason returns home and see Fred and Buzzy in the mirror. Fred tries leaving the mirror dimension by grabbing Jason's arm, but fails and Jason falls on the floor. A Mirror Monster tries luring him by disguising himself as a reflection of Jason, but Fred warns him from inside the mirror and the Monster takes his true form, that of an hideous creature with crab-like claws. The Monster drags Jason in the mirror dimension and tells him he will take Jason's place in the real world, but Jason manages to escape.
During the night, the Buzzy doppelganger manages to escape the garage by chewing through the door. The next morning, when Jason notices Buzzy's disappearance, he goes in town looking for Buzzy, but instead find Mirror Fred lifting a car over his head. Jason finally understands his dog and his best friend were replaced and decides to return in the mirror dimension in order to free them.
The mirror opens late at night and Jason enters. He falls into a hall of mirrors and sees Fred and Buzzy. However, Jason can't seem to break the glass to get out. He pulls out a pocket mirror and tries to use it to break the glass, but this doesn't work. He ends up pointing the small mirror at Fred, which clones the boy. Jason creates five clones of Fred, but even their combined effort can't break back into the real world. Jason points his pocket mirror at the original mirror, and the glass begins to sizzle. Jason, Fred, and Buzzy fall into the real world, but the Monster that wanted to replace Jason appears and tries to attack Jason. Jason rips out the creature's eyes and shoves the creature into the mirror dimension. Jason then picks up a lamp and hurls it at the mirror, destroying the dimension's entry point. However, the Mirror Monsters impersonating Fred and Buzzy are still outside. Furthermore, a hairy snake-like creature comes out of the dresser and asks if his note was received (at the beginning of the book, Jason had found a note warning him about the danger of the mirror).
The Mirror Monsters are described as hideous humanoid creatures, with claws similar to that of a crab and big protuding eyes. However, they also have the ability to disguise in their victims (it is unknown if they can transform in the real world, since we only see they transform in the mirror dimension to trick their preys or to replace them).
The Mirror Monsters seem able to imitate the personality of whoever they replace, at least to a certain extent. However, the doppelgangers are generally more aggressive, and physically stronger, than the real persons, despite being easily overpowered by a 12-year-old boy. The Mirror Monsters are also cunning and manipulative, and determined to replace people and join the real world.
- Despite not being about ghosts, the book cover depicts a ghosly hand emerging from a mirror. It could be that Tim Jacobus only knew the title of the book, and didn't manage to read it before illustrating it.
- The Mirror Monsters are similar to the Reflections from Let's Get Invisible!. Both books depicts monsters from a dimension inside a mirror trying to replace the real people. However, the Reflections seem to be truly sentient reflections (and not disguised monsters), being inverted. Moreover, to replace people, they have to wait people to use their mirror to become invisible, while the mirror of Ghost in the Mirror can't turn people invisible.
- A Reflection is also pictured in the short story Mirror Mirror on the Wall, from More & More Tales to Give You Goosebumps. In this story, the mirror can't turn people invisible and the Reflection can leave the mirror without outside influence and use any mirror to get out, so it is unclear if it is a Reflection or a precursor of the Mirror Monsters.