Idea for a book?
A child finds a small armillary sphere - a model of the sky made of interlocking bands of metal - in their grandparent's house. They play with and, by mistake, set it to the right combination. The sphere is, in fact, a puzzle, if set to the right arrangement it unlocks a door to a parallel dimension. It is, in effect, an invitation from the inhabitants of that dimension, dropped through into our to find the brightest humans and them into this other world. The child discovers that one of those humans has taken over the parallel dimension and is planning to invade the Earth. The child is rescued by the resistance and becomes involved in their attempts to rescue not just their reality but the Earth's too.