Amanda, who is somewhat of a bookworm, is forced to live with her Aunt Dottie, and her teenage cousin, Beth, because her parents were busy at an archaeological site. Before she goes inside, she notices the house next door and feels very strange about it. Once she meets her cousin Beth for the first time, she decides that she would have had more fun looking at rocks with her parents. Beth is snotty and rude, and demands that Amanda do her chores, or she won't let her hang out with her and her friends. Amanda agrees mostly because she really has no choice.
Later on, Amanda meets the family's nanny. A troubled old woman who seems terrified of the house next door. She also notices a gold locket the woman carries with her all the time. Beth has decided that the only way Amanda can be in her group is if she is able to spend the night alone in the house next door, which she explains is haunted, by the ghost of a girl who was chased there and locked in her room, until she starved to death. Amanda notices how mean Beth acts to all her friends and contemplates whether or not she even wants to be friends with her, but after much teasing, she decides to go inside.
She walks slowly into the room in which the little girl is said to have died. She notices some writing on the wall but can't read it because it's backwards. She looks at it through the mirror across the room and is startled to see that it says HELP ME! She looks in the mirror and doesn't see her own reflection, but the reflection of the little girl's long lost spirit, who seems to be trapped in the mirror.
Amanda runs home, and tells Aunt Dottie everything. The girls are forced to go back to the house to clean off all of the writing, but now the words HELP ME written backwards, cover every square inch of the room. The girls begin cleaning, each thinking the other one did the writing to get them in trouble. Beth is distracted by something in the mirror, and accidentally walks right into it. She disappears inside, and in her place is the little girl's ghost.
The little girl, holds up a gold locket much like the one that Beth's nanny had, and Amanda puts two and two together. She tells the girl that she can help and runs over to Beth's house. She looks everywhere but can't find the nanny. She sees her getting in to a car with all of her bags backed, planning to leave. Amanda stops her just in time, and brings her to the house, by showing her the locket. They arrive in the room and the nanny and her daughter have a tearful reunion as they both enter the mirror, to spend the rest of eternity together. Beth's friends come over to see what Beth was up to. Beth hadn't come out of the mirror yet, and was screaming to be let out. Amanda made her promise to be nice to her friends and to not boss everyone around. Beth agreed and Amanda, had an ok Summer after all.