Bosom friends Filka and Petka were at a loss. In one of the boxes with manuals for the biology class, which the head teacher ordered them to carry into the classroom, lay... a skeleton! But where did the strange ring come from on the bony finger - exactly the same as on the stuffed raven from the other box? And who was the mysterious invisible man who said the ominous words: “Soon you will pay for this!”? Without thinking twice, Filka decides to deal with this devilry and resolutely reaches out to the terrible find. And then a grinning skull...