BBC于2013年8月4日宣布彼得·卡帕尔蒂接任博士，将在2013年圣诞篇中取代马特·史密斯重生为第12任doctor。 第八季其总编剧依然为史蒂芬·莫法特。
A dinosaur materializes in Victorian London and spits out the TARDIS onto the banks of the river Thames, from which emerges the freshly-regenerated Twelfth Doctorsuffering from post-regenerative traumaand Clara Oswald. They are greeted by the Paternoster Gang, comprising of Silurian Madame Vastra, her human maid and wife Jenny, and Sontaran butler Strax. The Gang instruct the police on how to calm and contain the dinosaur and take the Doctor and Clara to their residence, where the Doctor rests and Vastra confronts Clara about her prejudiced attitude to the Doctor's changed face. Clara admits that she is struggling to adapt to the new Doctor, due to his stark difference to the old one, particularly his apparent age. The Doctor awakens to hear the dinosaur's pleas for help before it spontaneously combusts. The Doctor joins Clara and the Paternoster Gang at the river, and learns from Vastra that this is not the first incident of spontaneous combustion recently. The Doctor spots an unfazed man across the river and jumps in to investigate. Clara returns home with the others to start their own investigations hoping they will cross the Doctor's path.
The Doctor roams back streets for answers the next morning, questioning how his body chooses a new face during regeneration. Both Clara and the Doctor find a message reading "The Impossible Girl" in a newspaper, directing them to a restaurant where they reunite, but learn that neither planted the message for the other, as they had both assumed. They realise that the room is filled with robots to prevent them leaving. They are descended into a lair where they see the man from the river surrounded by other, idle human-like robots. The Doctor surmises the man is a long-lived cyborg, but in the unusual sense that it is a robot trying to make himself human by replacing his parts with biological ones taken from the victims of spontaneous combustion, including the dinosaur.
The Doctor leaves Clara as the robots and the cyborg begin to awaken after escaping himself. Recalling his earlier advice, Clara holds her breath to hide herself from the robots, but she is soon captured and taken to the cyborg. He states that he is trying to reach the "promised land" by prolonging his life via parts replacements and questions her on the Doctor's whereabouts. The Doctor returns along Paternoster Gang to fight the robots with Clara whilst the Doctor chases the cyborg back to the restaurant. The cyborg releases a hot-air balloon made of human skin, lifting the restaurant into the sky with him and the Doctor inside. The Doctor interrogates the cyborg, who reveals he and parts of the restaurant are from the SS Marie Antoinette, the sister ship to the SS Madame de Pompadour ("The Girl in the Fireplace"). The Doctor warns the cyborg that he will either have to kill himself, or he will kill him to protect the humans. Moments later, the cyborg has fallen from the balloon and is impaled on the spire of Big Ben. Clara and the Paternoster Gang see the other robots go lifeless after his death.
After regrouping, Clara finds the Doctor and the TARDIS have vanished. Vastra assures her that the Doctor will return, pointing out that Clara as the person that knows the Doctor the best is already prepared to leave. The Doctor reappears, planning to take Clara home, but instead arrives in Glasgow, where a phone call from the Eleventh Doctor to Clara moments before the regeneration from his perspective encourages her to stay.
In the epilogue, the cyborg awakens in a beautiful garden, and meets Missy, a mysterious woman who claims the Doctor as her boyfriend. She tells him that he has reached the promised land at last.
