Bert Large has been hiding a heartbreaking secret from his son Al, which threatens to jeopardize their close relationship. His reluctance to produce his son's birth certificate arouses Al's suspicions. Could it be that Bert is not his dad? Martin visits the Flint brothers at the remote country home they share with their father Victor and mother Doreen. Wallace and Paddy Flint have been suffering chronic stomach disorders, and a blood test confirms they have salmonella poisoning. But there's something else odd about this family. Meanwhile, PC Mark Mylow has had to deal with his domineering sister's arrival in Portwenn. She has decided to set up in business in Mark's flat above the police station as a herbalist offering alternative remedies.
Doc Martin: Blood is Thicker, Thursday at 9 p.m. on PBS 6.