While visiting the house of Max Thompson on his birthday, Erin and her friend, April discover an old mirror hidden away in Max's attic. When Max pulls the chain of the light overhanging it, Erin is shocked to see he's vanished. Upon pulling the chain again, Max reappears, confused as to his friends' shock.
Erin believes the whole thing to be an illusion until she returns another day to see the mirror. When she and April go back up to the attic, they find Zack, another of Max's friends, now knows about the mirror, and wants to start a competition of who can stay invisible the longest.
As the competition heats up, the time spent by each person as invisible grows. Max begins to grow cautious of the mirror's potential danger, but Erin and Zack only seem interested in beating each other's score.
When Erin goes invisible for a long time, she comes back different. She and Zack urge Max to go invisible once more. Upon his reluctance, Max is forcefully pushed towards the mirror before his friends turn on the light. While invisible, he meets his evil reflection, who tells him Zack and Erin have been replaced. The reflection attempts to switch with Max, but he manages to escape back into the real world.
Before the reflections can get him, Max's brother, Lefty breaks the mirror with a softball, setting the true Erin and Zack free, and destroying their reflections.
Erin is a fast talker, and her high pitched, squeaky tone has earned her the nickname "Mouse." When she finds the mirror in Max's attic, she becomes obsessed with it.
The book describes Erin as tiny, with short blonde hair. In the episode, she appears to be Asian.
- The Goosebumps Presents book states that her last name is Murphy.