Reg Trotter, absentee father to Del and Rodney, unexpectedly arrives to celebrate Christmas with his boys. Reg explains that he has been living in Newcastle, and after a visit from the infirmary, was diagnosed with hereditary blood disorder. Scared for their well-being, he decided to tell his two sons. However, several tests later, Reg is given the all clear but it seems Del Boy and Rodders have different blood types. It would seem the wayward father has some explaining to do. Reg explains that he frequently argued with their mother before they split. She had several dalliances with other men, and Del is the mystery child. Fortunately a visit to the family quack gives him news to put the smile back on his face.