The Secret (1990)

Sam and Amanda are friends. One day, they accidentally throw Sam's sister's diaries in the garbage can and have to trace them to the city dump. While looking for the diaries, they discover a secret passage leading from the city dump to a big, old, abandoned mansion nearby. There, they meet Valdemar, an old man who's been living there for sometime. At first, he's a little suspicious of them, but after a while, the three become friends and experience many fine adventures. However, they must keep it a secret, which is difficult, when their class mates start wondering what Sam and Amanda do after school. They also get to know Valdemar's sister Greta and hear about her dream; to see their childhood home once more before she dies. They decide to help her with that, which is easier said than done.