Nicholas Morgan's parents come home with their new baby girl, Hannah. Despite a small brown heart-shaped birthmark on her cheek, everything seems normal to the rest of the family, but Nicholas begins to suspect there's something wrong with his new baby sister, and that the birthmark is a "mark of evil". Strange things start to happen around the house regarding Hannah. Her eyes change color, her heart-shaped birthmark grows, and she eats his homework. He comes to believe that Hannah is a monster, despite his parents' assurances that there is a perfectly normal explanation for everything.
Things come to a head when Hannah attempts to stab him with scissors but instead Mom scolds Nicholas for allowing her to get near them to begin with. Eventually, it is discovered that while in the hospital, Hannah was accidentally switched at birth with the boy's real sister, and she is not related to him at all. She is returned to the hospital and they get their real baby, who they name Grace. She seems totally normal. When Nick's parents put Grace to bed, Nick gets up to go tickle her. Grace gets angry and says "If you tickle me again kid, I'll rip your arm off!" As Nick flees, he hears her say, "I'll get rid of you creep. Just wait until I can walk!'"
The episode follows a similar formula to the short story, only Hannah is named Grace instead. She can speak perfect English and constantly threatens Nicholas. She also continues to cause mayhem that gets Nicholas in trouble. At the end of the episode, she is given back to her parents, a pair of monsters, and Nicholas' real sister, Chloe, is returned, who can also talk and threaten him.
Despite being a monster, Hannah looks like a caucasian human baby girl with one major exception, a brown heart-shaped birthmark on her cheek that Nicholas believes to be a mark of evil.
Hannah is pure evil. Loving to threaten Nicholas whenever she can while also trying to either kill him or get him in trouble depending on the situation.
- In the original story, the monster baby's name was Hannah, while Nicholas' real sister was named Grace. For unknown reasons, the television adaptation named the monster baby Grace and the real sister Chloe.
- Being a one-year old rather than a newborn baby, Hannah is the youngest Goosebumps antagonist.