When Simon and Katie are playing in the field near their home one day, they see a large cloud floating overhead. When they notice a boy looking down at them and waving, they can't quite believe their eyes. Things become even more unbelievable when he floats down to them in a cloud basket and tells them that they have been chosen to be Cloud Children who can travel the world and help people in difficulty. They would come home with new friends, new experiences and even a pet dog. This is the story of their adventures.