Magician's Nephew

In early 20th-century England, neighbors Polly Plummer and Digory Kirke look for adventure during their summer holidays.  They live in a row of connected houses and decide to travel through the attic passages to get into a vacant house and explore it.  Instead of discovering an abandoned house, the children stumble into a forbidden study room that belongs to Digory's Uncle Andrew.  When Uncle Andrew discovers them, he pretends that he is going to keep the children captive, but relents and says they may go.  As Polly is leaving, he gives her a present of a yellow ring.  Polly vanishes along with the ring.  Uncle Andrew tells Digory that Polly has been sent to another world, and only way Digory can bring her back is to put on a yellow ring himself and follow her.  Digory is also given a pair of green rings, which are supposed to bring them both back home.