With Charles away, Caroline has a lot of extra work on her hands. Mary is doing extra work too as she wishes to study for a special examination at school with the prize for the winner being a beautiful new Webster's Dictionary.
In order to cram in extra study, Mary sneaks out to the barn so that she doesn't wake her family. Unfortunately, while she is asleep, she accidently knocks over her lantern and sets fire to a part of the barn, burning several important things, including an expensive history book that Miss Beadle has loaned her to help with her preparation.
Caroline is terrified and furious and, in her anger, forbids Mary to take the examination. Poor Mary is devastated about the whole thing and takes a part-time job at Oleson's Mercantile in order to earn some money to replace the book and the things from the barn. She also decides that she will take the test even though her mother has forbidden it.
On test day, Caroline sees Mary doing the examination with the other children but she does not win the prize. Miss Beadle informs her that Mary wrote her a note saying that she wouldn't be taking the test and why.
Kudos to Melissa Sue Anderson here. I always felt that, through no fault of either young actress, she was very much in the shadow of the equally talented Melissa Gilbert, at least in the early series of the show. In this episode, she was really able to show what she could do and she came through with flying colours.