Nick and Sally need a strong piece of string to pull their train, but it breaks, and they don't have anything stronger in their craft box. The Cat takes them away, but the Thinga-ma-jigger malfunctions and they have to make an unscheduled landing. It turns out they've shrunken and are inside the craft box, where they discover that they can create a stronger string by braiding several strands together. Nick and Sally are in the treehouse making funny faces at themselves in the mirror when they notice a bird flying back and forth into the tree branches. They discover that the bird is building a nest and they want to get a closer look but can't do it without disturbing the birds. The Cat takes them to a mirror world where they learn how reflections work and they concoct a kind of periscope that allows them to watch the bird without disturbing it.