Miss Beadle hands out report cards at school. Willie and Nellie tell the Ingalls girls that they get a nickel for each "B" and a dime for each "A." For once, Laura has done really well, and Mary has not. They bring their report cards home, and Laura gets affirmation. Mary is frightened, but Charles reacts to her grades pretty evenly, since he knows how hard she has been working. Relieved, she goes to bed. However, Caroline knows that something is wrong, and tells Charles that Mary has been working harder than ever. She can't understand the poor grades.
One day, Mary tells Pa that she's not feeling well and needs to stay home. Meanwhile, Caroline goes to the Mercantile and Harriet eggs her on about Willie's good grades, Nellie's achievements, and the history exam that is taking place that day. It is then that Caroline realizes why Mary feigned illness. Miss Beadle can't understand what is going on either, and an emotional Caroline resolves that Mary will just have to work harder.
After another poor day at school, with Mary unable to give a basic answer from the blackboard, Pa finds Mary that night working late on some math that should have taken her only a few minutes. When he tries to help her with the problems from across the table, Mary tearfully confesses that she can't see the slate.
Taking her to Mankato, Charles brings her to an optometrist. Mary has her eyes examined. She is fitted with a new pair of glasses, and to her delight, realizes that she can see everything for the first time in a long while.
Mary's delight is short-lived when her classmates tease her the next day about her new look. Led by Nellie and Willie, they chant "four eyes, four eyes." Miss Beadle subdues their mockery by putting on her own spectacles. However, they won't leave her alone, and Mary wonders if she'll lose her popularity and be an old maid like Miss Beadle.
When she can no longer stand the teasing, Mary "loses" her glasses by leaving them in the hollow of a tree. Charles is disgusted because he won't be able to afford a new pair until the next season. However, after Mary witnesses Miss Beadle with a handsome beau, Mary's fears of being a staid old maid disappear. She runs back and retrieves her glasses. She wins the history award, but is miserable because she knows she must tell her Pa the truth. She tells him about hiding the glasses and why, crying.
Pa says he kind of suspected it all along. Forgiving her, he hugs her and tells her he understands how badly words can hurt. They head home to show Ma.