Bettina from next door is afraid of everything and everyone, especially of the big, dangerous dog known as the black trash can. She watches him repeatedly and dares not come close. But when Bettina frees the black trash can from the thorny bushes in front of the house where he has gotten tangled, the two become friends. The dog dashes off, leaving behind his chain, a protective and strong chain that has seen a lot. Bettina wraps it around her body and from then on becomes the brave Betti Chainmail. Together with the black trash can and the partridge Tek-tek, she lives in the territory of Dr. Müller-Meckel. He finds disorder a thorn in his side and tries everything to get rid of Betti. But not even the worst nastiness can harm Betti.