Puppy girl Lotte and traveller dog Klaus must solve a great mystery.

One night two strangers creep into Gadgetville, searching for a suitcase belonging to the old traveller dog Klaus. Puppy girl Lotte surprises the strangers who become frightened and flee, dropping the yellow stone they had taken from the suitcase. Klaus found the stone on his last journey in the mountains. Actually there were three stones; the other two were left to Klaus’s friends, the dogs Fred and Ville. Lotte discovers that this is not an ordinary stone. There is a complicated mechanism inside it. Klaus and Lotte decide to find Fred and Ville in order to get the other two stones and solve the mystery. Two strangers follow them. Soon there is another competitor interested in the stones – Paul the Cat. Lotte realises that this is much more serious than an ordinary exciting adventure.