The Doctor (Peter Capaldi) saves the soldier Journey Blue (Zawe Ashton) when her ship, The Aristotle, is attacked by Daleks. He returns her to her vessel, which is a hospital ship become a warship because the Daleks don’t leave wounded. Aboard are devices in which doctors can be miniaturized and injected into patients. They show the Doctor their most interesting patient—a Dalek.

Danny Pink (Samuel Anderson) a former soldier suffering from PTSD, is teaching math at Coal Hill School. Clara (Jenna Colman) invites him out for a drink. The Doctor speaks to Clara about the damaged Dalek found by the human rebel ship Aristotle in the future. The Dalek insists that all Daleks must be destroyed. Clara agrees to come along when the Doctor goes to check it out.

The Doctor, Clara, and three rebel soldiers are miniaturized and enter the Dalek, whom the Doctor calls Rusty. They arrive at the cortex vault, which usually supresses all thought of compassion. Rusty talks about the beauty of the cosmos he has seen and says that the Daleks should be exterminated for wanting to destroy that beauty. The Doctor repairs a damaged power cell while he’s there but finds that it returns Rusty to the usual Dalek thinking pattern.

Rusty contacts the mother ship and launches an attack. From inside the cortex vault, Clara awakens Rusty’s memory of a star’s creation. The memory shows Rusty the beauty of the universe as he has seen it, but he also assimilates the Doctor’s hatred of the Daleks. It begins to exterminate its fellows. The Doctor is disturbed that Rusty absorbed so much hatred from him. It sends a message to the Dalek ship that the Aristotle is destroyed, and the attack ends, but Rusty rejoins the Daleks, perhaps as a fifth columnist. Journey Blue wants to travel with the Doctor, but he wants nothing to do with warriors. Clara returns to the school and dates the ex-soldier there.

The story, of course owes a great deal to Fantastic Voyage. Rusty says, “Resistance is futile,” which was first spoken by the Cybermen in a Doctor Who episode in 1967, and then appropriated by the Borg in Star Trek. The episode begins a long-time inner dialog on the part of the Doctor: “Am I a good man?” Like the best Doctor Who, it is filled with action, and yet thoughtful.