Little House on the Prairie S04 E21 I'll Be Waving as You Drive Away (1)

"Together, a pioneering family faces a crucial test of its courage and its love!"

Mary falls in love with Seth. When Mary's eyesight starts to get worse, Charles takes her to a specialist. He is devastated when the specialist tells him that Mary will lose her sight. Charles is unable to tell Mary for a while that she is going blind. When Mary does go blind, the whole family suffers.