It is another beautiful summer in the village of Portwenn, and arriving into the bustling village is a surprise that will turn Doc Martin's world upside down: the arrival of his long lost brother. However, like any good Portwenn mystery, neither Martin nor his long lost brother have any clue they are actually related (although it becomes rather obvious shortly after his brother's arrival into town).

Doc Martin's brother is Dr. Nathaniel "Nathan" Parker, a university lecturer from Manchester with a Welsh PhD, who becomes the new head master for the Portwenn School, taking over Lousia's responsibilities who is now committed to raising James Henry as a happy (or unhappy, as one will soon find out!) homemaker. Nathan moves to Portwenn after leaving university lecturing life due to the fact that he has been suffering from a medical condition causing him to "pass out" during his lengthy lectures at the University of Manchester. Doctors in Manchester are not 100% sure of the cause of his "passing out", so they recommend he simply re-locates to a smaller, quieter, slower pace of life that can be found nowhere else but of course, Portwenn.

Doc Martin and Nathan first meet when they accidentally literally "run into" each other. Nathan is busy griping to a colleague on his cell phone about his relocation to Portwenn, when Martin comes running around the corner with his briefcase on the way to an emergency and runs smack into Nathan. Nathan, irritated at the incident, yells at Martin, "Hey, Watch where you're going!". Both exchange the identical disgruntled glance at each other which would cause onlookers to not only crack up laughing at the exchange, but also to begin to sense that there is somewhat of an interesting resemblance and connection between the two.

Nathan, "Dr. Parker" , adorned in a top notch "Saville Road" suit with a handmade silk tie and matching braces, polished handcrafted leather shoes, and outfitted with a state of the art iPhone, iPad, MacBook Pro, Apple Watch, and tortoise colored glasses, is not at all thrilled about his "downgrade" from his lofty academic career in a bustling and technologically advanced city to re-locate to such a small village and "simple" job as a village school head master, and he makes his reservations clear as he is relocating to Portwenn. He shows the same blunt, grumpy disposition Doc Martin showed upon his arrival into Portwenn, and he manages to "tick off" the entire village shortly upon his arrival into Portwenn. When visiting the Chemist, he shows Mrs. Tischell the same cold treatment Doc Martin has continually shown her, which she finds rather intriguing about him, and his first remark upon meeting Bert and Al at Large Restaurant is a blunt, "What is this, Mutt and Jeff?". He is also an extremely finicky eater, not touching shellfish or red meat and only wanting chicken, turkey, lean fish, and vegetables, stating that the rest of it is all "nasty". Another issue with Nathan's eating habits is that he seems to go extended periods of time without eating a decent meal, engaging himself in lengthy periods of work without stopping to break and eat, saying that he has "no time to indulge in physical food when there's people needing food for their minds" (education). He seems at his best stuck behind a desk involved in academic research for hours at a time. His unhealthy skipping of meals seems to catch up with him rather quickly.

What irritates Nathan most about Portwenn is the fact that the villagers do not seem to "respect" his title of him holding a doctorate degree clearly enough. He becomes increasingly frustrated and irritated when the town continues to refer to him as "Mr. Parker", and he is quick to correct them with what will become his famous catch phrase around town, "That's Dr. Parker. I have a PhD". The village also begins using his famous catch phrase as an attempt to poke fun and mockery at him (except for the ones that respond "A Ph What?"), frustrating and infuriating him even further.

Nathan is even approached by PC Penhale while around town, with PC Penhale telling Nathan that the town has not been pleased with the way he has been treating people, and that he needs to show a little more kindness to the villagers. Nathan responds with a blunt: "Are my actions a criminal matter?" PC Penhale responds with "Well, no". Nathan responds with: "Then it's not your jurisdiction. Being an (insert cough cough word here) isn't a crime".

One interesting tidbit about Nathan's first arrival into Portwenn. He never learns Martin, Lousia, or Ruth's surname at first, which turns out to be a good thing for now (there is excitement in why later). He hears someone refer to Martin as "Doc Martin", so he naturally assumes Martin's surname is actually "Dr. Martin". Martin also discovers later on the fact that they refer to him as "Doc Martin" actually might not be a bad thing after all.

Meanwhile, Louisa is "settling down" to the "happy (or should I say unhappy!) homemaker" life raising James Henry and being the "model wife". She reads some articles in "Today's Homemaker" magazine and tries to put some of the suggestions into practice . She burns supper and makes a complete mess in the kitchen (and all over her brand new outfit she wanted to "impress" Martin with) with her rice boiling over. Her vacuum cleaner bag explodes and sends dust flying through the upstairs. And through it all, James Henry cannot seem to quit crying long enough for Louisa to complete her house chores. Nathan drops by the back of Martin's office near the kitchen to pick up some paperwork from Louisa for his new position as the school's head master, where he sees Louisa in a mess, and James Henry crying. Although Nathan's extremely blunt and grumpy as an individual, he tends to show a soft spot toward James Henry and assists Louisa in comforting down James Henry so he stops crying. He may not be able to stand much of the Portwenn village, yet there is a little connection between James Henry and Nathan. Louisa even jokingly refers Nathan to James Henry as "Uncle Nathan", which seems to not set too well with Nathan.

Nathan also gets a chance to meet Ruth, and as Ruth sits and analyzes Nathan's actions (as any good psychiatrist does), she begins to sense something about him. She recalls a long lost family secret her brother told her years ago about the time he spent apart from his wife while they were separated. She fears that family secret may have just arrived in Portwenn.