Clara and the Doctor get a call from Rigsy, who we met in Flatline from Series 8, in the TARDIS. He has a tattoo counting down. They realize Rigsy has been on a lost street in London. They discover an alien refugee camp led by Me (Ashildr). She is Mayor and in order to keep the peace she has strict rules. Rigsy has been sentenced to die, for allegedly killing someone. There is no real proof, but in this lost street circumstantial evidence is all that is needed. These are harsh rules to enforce the peace.
A theme that is repeated in this is Me needing to protect everyone else from the Doctor. These refugees, it is intimated, are the fallout from the Doctors actions. They are the remnants of his dealings with various alien races. Me is willing to do what ever it is necessary to keep the peace and protect the community she created.
On the other hand the Doctor is there to save an innocent. He is shown to be the righteous protector. The episode also follows the concept of whether or not the Doctor goes to far to protect those under his direct protection. It also looks into the fallout of that too. The Doctor rarely stays to see the consequences on others of his actions. That is why Me has taken it in her power to help. She saw the results of his good deeds.
After an episode I found disappointing, this brought back the energy and tone I missed last week. Typical Doctor Who, it follows the old quote from Arthur C. Clarke, “Any sufficiently advanced technology is indistinguishable from magic.” Many of the alien technologies seem magical. The writers do not even try to explain it. They let the story do that. Overall I liked this, so it gets a B+.