During a two week break from school Mary heads to Mankato to get an update on her eyeglasses' prescription.  Which only solves the issue for a moment...until Mary says her new glasses aren't any better than the old ones.  While Mary continues on about the shadows appearing on the pages in the books she's reading, the residents of Walnut Grove, or more exact those who are members of the grange,  are in for a world of hurt when the Railroads decide to retaliate and break-up the grange.  Meaning anyone who has crops to sell has ZERO way of getting them to markets. Meanwhile, there's a new family in Walnut Grove with a son named Seth Barton who's taken a liking to Mary...even though Laura saw him first.  After a second trip to the optometrist in Mankato, Charles learns the truth that Mary's vision will eventually disappear. He shares the news with Caroline, Doctor Baker and Reverend Alden before he shares the news with Mary. And it's not long after the news is shared that Mary wakes up to find her world has gone dark.   Unsure of how to help Mary adjust to living in her new world, Charles and Caroline decide to send Mary to Iowa to attend a school for the blind in the hopes that Mary will find her way once again into the light.  Why did we even need Seth Barton in this episode?  Shouldn't Mrs. Simms remove her newborn out of the house if her stepson Lucas has a a highly infectious disease? What's to become of Walnut Grove if no one is able to make a living?