The Telegraph felt this week's monsters were suitably scary "In a classic case of keeping the monster unseen for as long as possible, we didn’t get to meet this story’s antagonists until past the episode’s halfway point. When we eventually did, they were suitably terrifying: clad in blood-red robes, with creepy claw-hands and zombie-esque faces which recalled The Mummy film franchise."
While The Mirror enjoyed the episode, it found the script to complicated. "The problem with Extremis isn’t in the watching of the episode. The problem with Extremis is afterwards, when you stop and think about the episode. Yes, I’ve got a case of Moffatitis"
Digital Spy agreed the plot was to difficult to follow. "This week's outing feels like a step backwards. Starting and ending in two totally different places, and leaving the audience baffled in between, It's anything but straightforward, going against the back-to-basics ethos that previous episodes have adhered to."
The script was not a problem for IGN, which enjoyed the complex nature of the story. "Moffat jumps around quite a bit with “Extremis,” aligning a variety of elements to get this first part of the story off the ground, but of course the return of Michelle Gomez as Missy -- and the revelation that, yes, it’s her in the vault -- is of particular note."
Den of Geek also enjoyed the story as a prelude to a multi-episode tale. "Extremis isn’t action-packed, isn’t jammed with effects, and doesn’t need extensive explanations. Its idea is in fact beautifully simple: it’s a dry run for something very big, and very nasty."
AV Club called the episode a great, experimental Doctor Who and in particular praised the lead actor. "Peter Capaldi is perilously close to becoming my unqualified pick for favorite Doctor, and the overriding reason is on display as he gently breaks it to Bill that neither of them nor anything else in this world is real. He underplays the moment, making small choices to signal both his compassion and his heartbreak."
Screen Rant admired the premise of the story, "The hour has fun with its exploration of the Truth and in slowly pulling the rug out from under both audience and character. The reveal that the Doctor, Bill, and Nardole are in a massive computer simulation meant to test possible outcomes for an imminent alien invasion gives Moffat the chance to deliver a handful of delightfully unnerving scenes, culminating with a mass suicide at CERN"
Finally Games Radar thought the Doctor Who deosn't get much better than this. "When you realise that nothing you’ve seen is ‘real’, you see how fooled you were from the very beginning like the Doctor, Bill, and Nardole. You see that the suicides, initially looking like a bit of cheap intrigue, were a clue all along. The book Veritas isn’t just a plot device used to introduce the monsters, it’s the key to the entire episode."