Every once in a while, the Doctor sees a companion or good friend die, but he resists the desire to break the rules of time travel to fix it.

That’s why Katarina died in the 1960’s, Adric in the 1980’s,  Adelaide Brooke six years ago, and most recently Osgood. They met their destinies, and they had to be fulfilled.

So what happens when the Doctor finds a new way to help someone else cheat death?

That’s what happens in “The Girl Who Died,” featuring Maisie Williams from Game of Thrones. She plays Ashildr, a villager with a lot of imagination. She also makes the mistake of challenging some visiting space vikings called the Mire. After they take the warriors of the village, basically to harvest their testosterone and adrenaline to make energy drinks, she and Clara manage to get in their ship. Clara warns the head Mire viking, who calls himself Odin (David Schofield) that picking a fight with her and the Doctor would be a bad idea. It’s almost how the Doctor would approach it, again. Ashildr’s challenge, however, changes all that.

This puts the Doctor in a tough spot. He wonders if he can save the village, even without his sonic shades and TARDIS. If he does, it could cause a future where the Mire will be more interested in Earth as a point of conquest. If he does, people still die. As he tells Clara earlier in the episode, the basic rule in time traveling is treading softly, to cause slight ripples rather than tidal waves. So far, he has caused a ripple that permanently affected Daleks, then a bigger ripple that changed lives in a mining facility.

So how can he cause a ripple that only saves this village? He has to solve this thanks to Clara’s prodding, and a touching talk with Ashildr. Williams does a great job filling out her character as an odd and imaginative girl who is still loved in the town. That, to her, is reason alone to stay and fight. It’s also interesting that when they see each other for the first time, they act as if they’ve always known each other. The Doctor calls it “remembering in the wrong direction”.

The Doctor decides to do his best to train the remaining villagers, and even giving them nicknames like “Limpy”, “Chuckles” and “Heidi”. It doesn’t go well. However, thanks to translating a baby’s cry, he figures out the village has electric eels. He uses that to hold off the Mire through some clever science.

Ashildr is also part of the battle, as she uses a stolen Mire helmet to create a fake dragon through her imagination to scare the soldiers off. The Doctor, Clara and the villagers are so happy over their victory that they don’t notice Ashildr dying from the stress of using the helmet.

Fans know the Doctor feels pain when he loses a friend or Companion, but what happened here, and in the previous story, has apparently upset him even more than usual. Then, it comes to him: he realizes why he looks like Caecilius from “The Fires of Pompeii”. What he did during that episode, including saving Caecilius and his family from the lava of Vesuvius, reminds him that “I’m the Doctor, and I save people”. To those who disagree, “to Hell with you.”

That inspires him to change history in a new way. He takes a computer chip that’s used to heal a Mire warrior’s wounds, and places it on Ashieldr’s forehead. He’s hoping the chip will heal her, and it does. It’s the equivalent of turning back time to save Adric in “Earthshock,” which never happened. This is a big deal.

Thing is, the chip has more than saved her. It will protect her from disease and most anything affecting her health. She’s now the female version of Jack Harkness.  How will that affect her? As he tells Clara, “immortality is everyone else dying.” That’s why he left an extra one for her, for the person she wants to share immortality with.

Still, it looks like the Doctor will be facing a very big tidal wave when he meets Ashildr again, many, many years later.

Before the episode was shown, Williams did talk to the BBC about taking on the role, and what happens when fandoms collide:

She was also very excited about being offered the role. “I was going to do a tape, but ran out of time because I was in America shooting another film,” she said, “so my agent worked really hard and sent over lots of clips and videos – they liked them and gave me the role.”

So next week, we’ll see Ashildr’s new role, and how surviving the centuries has changed her.