Here be some serious spoilers folks.
We find ourselves in England during the 1600’s and Oliver Cromwell’s era, while a highwayman named the Nightmare robs a coach. During the robbery, we see the Doctor arrive and interrupt the robbery. The highwayman threatens to kill the Doctor for his interference. Neither the Doctor nor the highwayman get what they came for as the coach gets away. We discover the highwayman called the Nightmare is none other than Ashildr. The end result of the doctor saving her life was she became affectively immortal.
They both are after and amulet, the Doctor swears it is an alien artifact. Ashildr takes the Doctor to her Manor. There we learn she calls herself Me now. She has not been happy with her lot. She has become what the Doctor fears. She does not value life and keeps herself disconnected from other to keep from being hurt by the loss of people with shorter life spans.
We keep getting hints that Ashildr has an ulterior motive to get the amulet and a mysterious partner. The Doctor and Ashildr go to retrieve the amulet from the Manor of the lady that currently owns it. They succeed just barely. The Doctor decides to stay, because he knows things are not as they appear. Ashildr (Me) tries to convince the Doctor to take her with him. He informs her it is not a good idea. Ashildr admits she was not helping the Doctor, but the Doctor was helping her. She introduces her Cat-like alien partner Leandro. The Doctor deduces they did not ask for help directly, because they knew he would not approve of their plans. The amulet opens a doorway, but it needs a death to do it.
It looks at first it will be Ashildr’s manservant to be sacrificed, but they learn that a rival Highway, Sam Swift, is to be hung. Ashildr uses local soldiers to occupy the Doctor while she and Leandro escape to the hanging. The Doctor tricks his way to go after Ashildr and Leandro. At the hanging Sam uses jokes to delay the hanging. The Doctor gets in on the act and verbally spars with Sam. The Doctor uses his psychic paper to give Sam a pardon. Ashildr then slaps the amulet on Sam.
The Amulet begins to kill Sam in order to open a portal so Leandro’s people can conquer Earth. The destruction and cost of life is too much for Ashildr. She regains her humanity. She uses the device the Doctor gave her. This device was so she could turn a loved one into an immortal, the Doctor knew what loneliness could do. She never found anyone worthy. She uses it on Sam to save the Earth. This causes the portal to close and Leandro’s people kill him for failure.
The Doctor and Ashildr discuss things. She now knows the value of connecting with mortal people, but she also knows about those who are abandoned by the Doctor after he swoops in. Their lives are never the same. As the Doctor protects the Earth, she vows to protect the Earth from him. He asks if that makes them enemies. She says of course not enemies are never the problem; it’s your friends you have to watch for. She tells him she will be watching out for him. He tells her his is glad he saved her. We then go to the Doctor playing his guitar in the TARDIS. Clara comes in with news of one of her students that the Doctor impacted. As a thank you the student took a selfie. When the Doctor looks at the picture, in the background we see Ashildr.
A regular theme of Doctor Who is the fallout he causes in the lives he impacts, including the companions. Their lives are never the same they cannot return to “normal” life after all the wonders they experienced. Ashildr is an example of the Doctor running away from the fallout of his adventures.
Another theme is that immortality can cause a loss of humanity and a disregard for life, because the desire to shield oneself from the pain of being around those with such shorter life spans. This attitude is what the Doctor feared would set in when he gave Ashildr her immortality. In the episode he mentioned the other person who became immortal due to his adventure, Captain Jack Harkness. Like the Doctor he cares too much.
They have set up in Ashildr another foil to the Doctor. This one concerned about the wake the Doctor leaves when he goes. It will be interesting to see where this goes in the long run. I would love to see Captain Jack and River Song; both make appearances with this Doctor. Overall this story arc was great. This episode exceeded my expectations, so I give it an A.