Human Nature was the eighth episode of the third series of Doctor Who. It introduced the Chameleon arch and its ability to change Time Lord DNA, making the Doctor human. The episode was based on a novel of the same name.
The Tenth Doctor and Martha barely make it into the TARDIS as an energy weapon discharges behind them. After ascertaining that their pursuers had not seen their faces, the Doctor explains that they are being pursued by aliens who have stolen a Time Agent’s vortex manipulator and can follow the TARDIS anywhere. He gives her a watch and tells her his life depends on it…
In November 1913, at the Farringham School for Boys, schoolteacher John Smith wakes up from this dream. Martha comes in as his maid to bring him breakfast and addresses him as “Dr John Smith”. He explains to her that he sometimes has dreams of being an alien adventurer from another world, and she was with him. He notices the watch on the mantelpiece of his bedroom and claims he’s from the Earth, but she shows him a paper showing he’s in 1913 and he’s completely human.
Later, Smith runs into Matron Joan Redfern, the school’s nurse, and they chat about the boys. She wonders if he’ll be going to the local village dance and he rather awkwardly falls down the stairs. Martha comes to check up on him but Redfern casually dismisses her as a maid, under the assumption she has no medical knowledge. Smith talks about dreaming that he has two hearts but a stethoscope easily shows this is not the case. He shows Joan his journal of his various dreams and the creatures he has seen such as Daleks, Cybermen, and the TARDIS. He also has pictures of Rose, although she disappears from the later entries. Nurse Redfern leaves with the book and when Martha goes after her, she asks how Martha came to be there with Smith.
Meanwhile, a group of John’s students are studying in their dormitory room. One of them, Timothy Latimer, is bullied by a fellow student, Hutchinson, who compels him into doing his Latin homework with a daunting deadline—the next morning. He proceeds to bully Latimer further as much for his obvious intelligence as for his uncanny and unsettling ability to know things he shouldn’t or couldn’t possibly know, like where Hutchinson’s father is posted while serving abroad. Another student, Jeremy Baines, tires of teasing Latimer and announces to Hutchinson and the others that he knows the location of a secret stash of beer in the woods. He sneaks the window to bring some back for the boys to enjoy in secret.
Outside the local pub, Martha is complaining to her fellow maid, Jenny, that it’s not fair that they have to freeze outside just to have a beer. Her modern attitude amuses Jenny, who wonders where she gets such ideas. Martha then notices a green light shooting through the sky. John Smith arrives and greets them, explaining shooting stars to them. Martha doubts the explanation in this case, being exceptionally careful to protect him. She runs off to investigate with Jenny following her.
Baines has just found the cache of beer, in the woods near the school, when he sees a strange green light land on the field nearby. Curious, he investigates the light source, and stumbles across an entrance to the an invisible spaceship. Martha and Jenny arrive just moments after the ship briefly becomes visible, but miss seeing it as the cloaking technology turns back on.
Inside the ship, Baines talks with the spacecraft’s occupants, whom he can’t see. They call themselves the Family. He wonders why he cannot see them, to which the Family ask why he would what to see them. Baines answers that it’s because he wants to know what they look like. A raspy female voice explains that it can be easily done, as soon they will look very familiar. Baines screams in terror.
Hutchinson and the other boys are getting impatient back at the dorm when Baines returns, his search for the secret stash of beer apparently being “unsuccessful”. He is also behaving unusually, sniffing the air as though he has a cold. His eyes are also open completely, but none of the other boys seem to notice, saying there’s another stash somewhere else.
Martha rides out on a bike to an old hut. Inside is the TARDIS, on reduced power. She enters and greets the time machine, something that makes her think she is cracking up now. Martha thinks back to how the Doctor warned that their pursuers would never stop following them. He gave her a watch saying it contains “me,” and said the Family’s life spans were running out so they only needed to hide until they die. She remembers how the Doctor used a chameleon arch to rewrite his biology and transform himself into human. The TARDIS took care of everything else, creating a new identity for him. She turns on a recording the Doctor made prior to enacting his plan. She reviews it, but finds nothing pertaining to the strange sighting. She now worries that she may have to bring the Doctor back if they have found them, as it was the last resort the message said to use in that scenario.
Joan asks John to sketch her in his journal, being surprised at how beautiful she is in his eyes. They kiss as Martha walks in on them by accident. She excuses herself, mumbling that love wasn’t on the Doctor’s list. She finds refuge in the TARDIS, and reviews the instructions again. The instructions are notably lacking one thing: the Doctor did not foresee that his human self might fall in love, obviously something he never thinks of. However, she is more concerned about the strange green “shooting star”, something the Doctor didn’t mention either. On the subject about “John Smith” falling in love, Martha is annoyed and depressed that it wasn’t her he fell in love with.
Meanwhile, Latimer is summoned to John’s office to retrieve a book. While John looks for it, Latimer is drawn to the pocket watch that contains the Tenth Doctor’s Time Lord memories and abilities. The boy opens it, briefly seeing the Doctor’s world, and strange voices speaking of a “Time Lord,” but closes it quickly as he hears Smith approaching. Latimer slips the watch into his pocket and scurries away, unnerved. Also inside the school is Baines, who senses a trace of a Time Lord. He telepathically reports to the others the status of the Doctor’s whereabouts, and declares that it’s time to activate the soldiers. On nearby Oakham Farm, Mr Clark confronts a scarecrow that has just appeared on his property. He conceives it merely as a practical joke pulled on him by an one of the “idiots” at the school. When he pulls the straw out and sticks his hand through the other side, he finds they are just scarecrows. Many more animated scarecrows promptly surround him. He becomes the Family’s Father. Meanwhile, another young girl, Lucy Cartwright, is walking down a road when yet another scarecrow picks her up, frightening her. She becomes the Family’s Daughter.
Smith is giving weaponry training to the boys when the Rocastle, the school’s headmaster, arrives and orders firing to cease. Hutchnison tells him they could do “a lot better”, as Latimer is being “deliberately shoddy”, at least according to him. He denies this, telling the headmaster he is trying his best, to no avail. After some more conversation, Rocastle orders for firing to continue. Meanwhile, Latimer is haunted by a vision of the future where him and Hutchinson are under attack in the upcoming war. As Hutchinson’s patience is getting beyond the limit, he requests permission to take Latimer off for a “beating”. Per Smith’s countenance, the boys head back towards the school. Shortly after, Baines sniffs rather loudly, prompting Smith to ask if anything’s the matter. He quickly dismisses it, although not in a normal fashion. On a walk together, Redfern informs Smith that he wrote about a war starting the next year, but he dismisses the idea, hoping peace will reign. As he continues talking, he notices a woman and her carriage getting dangerously close to a piano on a rope. Instinctively seeking to save her, John grabs a nearby boy’s cricket ball and tosses it to start a chain of events that stop the woman just as the piano comes crashing down just feet away from her. Joan is impressed, but Smith says it was luck. He then asks her to the dance that night. Walking back to the school Joan tells John that she thinks that “the Doctor” is the kind of man he wants to be and his dreams are his subconscious’ way of trying to get to be so. They notice a scarecrow askew and John fixes it. Joan asks him where he learned and he reflexively says Gallifrey. However, when questioned about its location, John isn’t sure.
Meanwhile, the possessed Baines, Mr Clark and Lucy use their animated scarecrows to capture Jenny, who becomes the Family’s Mother. Jenny then returns to the school and engages Martha in conversation. Martha quickly realizes Jenny has been taken over. She runs to Smith, who is with Redfern preparing to go to the village dance that evening. She finds that the watch is gone, and tries in vain to restore him to his Time Lord configuration without it. However, Smith assumes Martha is delusional and thinks his fictional stories are real. Annoyed, Martha slaps Smith, telling him to wake up. Smith, shocked at Martha’s behavior, promptly fires her and leaves for the village dance with Nurse Redfern.
The Family break into Smith’s office and search it, as the scent is traced back to him. They find the flier for the dance and decide to follow. At the same time, Martha has arrived with the sonic screwdriver, prompting Joan to admit she feels something off about Smith. Tiring of Martha’s “games” is left speechless when she shows him the device he’s seen in his dreams. Latimer is still watching Smith from a distance, curious about him. The Family crashes the dance, demanding Smith tell them what he knows. However, Lucy/Daughter of Mine has been there as well and overheard Martha’s attempts to get the Doctor’s memory back, telling her Family the Time Lord is within their grasp.
However, John easily points out that he is human without any memories of the person he was before. Baines/Son of Mine is amused, seeing the Doctor went as far as erasing his memory along with changing his biology. As the Doctor is useless to them like this, they try forcing him to change back by threatening him. Remembering Martha said Joan was important to him, the Family takes both hostage and order John to choose between his friend and his lover.