Picking up about eight centuries from where the previous episode left off, we find the Doctor on the trail of an alien artifact, interrupting a robbery in 1651 England that is in the process of being carried out by none other than Ashildr (Maisie Williams), who by now has largely forgotten her name and now answers to "Me". Bored with eternal life, she has taken to fighting in wars and robbing the spoiled rich. The Doctor is astonished at this and resolves to help her rediscover herself.
Complicating things, of course, is a leonine alien who calls himself Leandro and who claims to have lost his family and his world, and wants to use the artifact to leave Earth. There's just one catch: the artifact, an amulet called the Eyes of Hades, requires the entire life energy of a living being in order to open the portal to Leandro's dimension. "Me" seems only too happy to help, and here is where the Doctor must find a way to convince the immortal girl of the error of her ways.
Although the episode was hit-and-miss, there were some decent enough bits that were worthy of consideration. For instance, Ashildr's body may be immortal thanks to the Mire medical chip embedded in her skull, but her memories are not—the chip suppresses memories that are too painful for her to bear, and there are a lot of them. In order to keep track of her own activities, she writes them down in journals, pages of which she tears out so she won't have to relive any events that are too traumatic. She only keeps intact the journal of how she lost her children in the Black Death to remind herself not to have any more.
By the end, of course, things work out and Ashildr declares her intention to remain behind and attend to all the people the Doctor leaves behind during his visits to Earth. That's to the good, because Maisie Williams is scheduled to make at least one more appearance in this series.
But beyond this bit of character building, the episode is fairly unremarkable. I give it a C-.