The Twilight Zone Recap: Little Girl Lost
This was one of the first The Twilight Zone episodes I ever saw - or remember seeing, at any rate. Perhaps that's one of the things that piqued my curiosity in things science fiction. Perhaps not. I just remember seeing it a very long time ago, though I probably would have said it was an episode of The Outer Limits instead.
In any case, two young parents are awakened one night when their daughter calls for them - but they can't find her anywhere in the house. Luckily a family member (a brother of the husband if I recall correctly) is a physicist or something who postulates an opening into the 4th dimension, which they are then - amazingly - able to find.