With its imaginative comic premise and its wide range of zany individual characters, It Runs in The Family picks up steam through its convoluted plot which mixes an illegitimate child, neurology, and Christmas. While preparing an important speech for a meeting of fellow neurologists, Dr. David Mortimore is interrupted by colleagues rehearsing for the Boxing Day pantomime and an unexpected visitor from his past – former nurse and extramarital lover, Jane Tate. Rather than returning to wish Dr. Mortimore a merry Christmas, Jane gives him a startling gift: their extra-curricular activities that had ceased over 18 years before spawned a boy, Leslie.