His green hat

It's been almost four years since the well known Moomin family moved out of Moominvalley, but when they come back, they find out that Snufkin had turned invisible. Normally, it would have had been a bit easy to get him back, especially since they've brought a child back from invisibility before, but due to how long Snufkins been invisible, it's going to be a bit harder for them to get him back.