Saturday's "Extremis," written by departing showrunner Steven Moffat, marks the halfway point of Doctor Who's reinvigorated tenth season, which means we're finally starting to dig into overarching storylines that will likely affect the rest of the season, beginning with who is in the vault.

Although we've yet to actually see inside the vault, from the flashbacks sprinkled throughout the hour and the Doctor's (Peter Capaldi) one-sided conversation that closes it out, it appears, as predicted, that Missy (Michelle Gomez) — or at least some version of the Master character — is the precious cargo locked behind the big vault door. And although we don't yet know why she was sentenced to be executed, we do know that the Doctor saved her and agreed to guard her body for the thousand years that followed.

Time Lord or not, a thousand years is a very long time to guard anything. We don't know how long the Doctor has been guarding the vault in his timeline, but it's no wonder he's been itching to get off-world since meeting Bill (Pearl Mackie). I'd be ready to bounce after 24 hours, so the Doctor is practically a saint in comparison.

Doctor Who isn't playing it safe anymore

But Missy's anticipated reintroduction to Doctor Who is only a small part of this week's episode. The bulk of "Extremis" builds rather slowly — it takes the majority of the hour for Nardole (Matt Lucas) to realize the entire adventure to the Vatican to read the mysterious Veritas has been a computer simulation — but by the end of the hour, it's clear that we're headed into unchartered territory. It's thrilling, to say the least.

Peter Capaldi, <em>Doctor Who</em>Peter Capaldi, Doctor Who


The Doctor, in his current state, is working at a severe disadvantage. He needs Nardole's assistance more than ever and refuses to tell Bill the truth about his eyesight. He says he doesn't want her to worry about him, but it's likely that Nardole's comments about how telling Bill actually makes it real is closer to the truth. Now, this isn't the first time the Doctor has been knocked down — despite everything he's accomplished over the years, he's often forced to battle his way back from perilous situations with the help of his friends. It's a great feat even if the show also manages to make it look easy. But the Doctor is damaged. He's not operating at 100 percent, which threatens to put the odds in his enemy's favor. However, the Doctor also makes a good point when he says, "You don't have to be real to be the Doctor. As long as you never give up. As long as you trick the bad guys into their own traps."

A recurring theme in Doctor Who is the idea that anyone can be a hero. The Doctor is a hero, but so is Bill, so is every single companion we've met along the way, so is every human or alien being who's ever stood by the good man in a fight. So, the Doctor and what he stands for will likely prevail, real or not. It doesn't mean this version of the Doctor will survive the oncoming war necessarily, but in the end, the good guys will win because they don't give up when they're down. They fight without an agenda, without the promise of a reward or recognition for their actions, because it's the right thing to do. That's what makes them the good guys.

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It's been quite some time since Doctor Who has been this strong, this captivating on a pretty consistent basis. This week's episode is essentially an hourlong appetizer to a main course that will be served next week, but I'm also ready to dig in now. The fact that "Extremis" isn't billed as a two-parter — maybe because the arc won't actually be wrapped up next week? — plays to its strengths by playing with our minds. Knowing ahead of time that this is an extended arc could have potentially alerted us to the fact there is something strange going on, but going in, ahem, blind puts us in the same position as Nardole, as Bill and as the Doctor. We are also simulations by extension, and so we are left questioning what exactly is real, too.

Moving forward, the Doctor is going to finally need to tell the truth — at least to his friends — about his eyesight. He's still the smartest person in the room, but his injuries also make him a bit more human, for lack of a better word. He's no longer the invincible Doctor he always appears to be, and he's lost in the dark as a result. Missy's appearance by his side is guaranteed to bring the show's energy levels up to 11 ("I've just been executed. Have a little respect."), but it's the chemistry and the complicated relationship between the two Time Lords that holds true power here.

Any time we get to explore the Doctor's past is a real treat — and there were certainly a number of references to the Doctor's past this week, from his relationships with Missy and River Song to his many, many sins over the years — but with the threat of danger coming in the very near future, the Doctor is going to need to rely on his friends in the present for help (again). Can we include Missy in that group? I guess only time will tell.

Doctor Who airs Saturdays at 9/8c on BBC America.