Once an excellent student, Miss Beadle, Charles and Caroline are concerned when Mary's once excellent grades begin to slip significantly. It doesn't take long for Charles to figure out that Mary is doing poorly with her studies because she can't see properly and an eye doctor in Mankato gives her a pair of shiny new spectacles that make all the difference. Most unfortunately, Nellie and Willie Oleson, among others, decide to start calling her 'four eyes' which makes Mary feel so terrible that she pretends she has lost her glasses. After some deep thought and a conversation with Miss Beadle, she realises she will never get her work done without them and miraculously finds them again.
Given what happens to Mary and her eyesight in later seasons of the show, this was an excellent piece of foreshadowing and very well acted by Melissa Sue Anderson.