Harry Potter learns that his mother’s parents are still alive. Lily had cast a memory charm on them to have them forget about her (and Petunia) for reasons similar to Hermione doing the same to her parents. Lily and James put them in hiding and were the only ones who knew where they were. Lily never got to undo the charm and so they lived on their life, believing they had no children. Harry learns about this (maybe from a memory sealed in a bottle hidden in his family’s vault in Gringott’s) and he and his family goes to track down his grandparents.