C4 - Cwn Cymru Cyfrinachol Cyf., a detective agency that use the skills of special dogs. Every dog is a detecive and are trying to catch the thief MM that stole gems from the Museum. Ffranco a Twm, Alun and Jac are caught and are detained in the cellar. Twm jumps at MM but is shot. The police arrive and Ffranco is promoted to C4 agent like Twm.