When Rodney opens the door to a stranger at the flat he soon learns from Grandad that it's his long lost father, Reg. As Del returns home from the pub, his anger is abated when Reg tells them he is ill and it may be hereditary. The subsequent blood test results threaten to split the brothers up.

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 to the infirmary, was diagnosed with an hereditary blood disorder. Scared for their well-being, he decided to tell his two sons.

Several tests later and it appears that Del Boy and Rodders have different blood types. It would seem the wayward father has some explaining to do - or rather, their late mother Joan does.

Del and Grandad recall Joan and Reg's frequent arguments around the time she fell with Rodney, and how she had acquired some new friends including a trumpet and saxophone player. They'd rather not think such things of Joan, but are forced to conclude that Rodney's a "whodunnit".

However, after a day out boozing, Reg has a revelation: Del is in fact the mystery child. But after the elder sibling spends much of Christmas week kipping on Trig's sofa, a visit to the family quack gives Derek news to put the smile back on his face.