Extremis (Doctor Who)

The Vatican calls upon the Doctor (Peter Capaldi) to investigate the Veritas, a book whose readers typically kill themselves after reading it.

In the present, the Doctor remains blind following the events of "Oxygen" and wears his sonic sunglasses to provide a limited form of sight while hiding his condition from everyone other than Nardole.

The Doctor is met by the Pope to help deal with a recently translated text called the Veritas, that causes readers to end their own lives.

Meanwhile, the Doctor temporarily regains his sight using Time Lord technology and tries to read Veritas, but is forced to flee into a second portal with the translation when the skeletal, corpse-like Monks appear.

A distraught Bill follows a trail of blood through a portal, finding herself in the Oval Office of the White House, where the Doctor is waiting.

The Doctor explains that Veritas describes a "demon" planning to invade Earth by creating detailed simulations of it to practice invasion.

Simulacra within these "shadow worlds" can discover the false reality as they can only come up with pseudo-random numbers, leading them to commit suicide to escape the simulation.

River Song's TARDIS-styled diary, first seen in the Tenth Doctor story "Silence in the Library"/"Forest of the Dead" (2008), is also featured in the episode.

[13][19][21] On Rotten Tomatoes, 93% of 14 reviews are positive, with the site's consensus reading "'Extremis' expertly juggles several of Doctor Who's more thought-provoking themes, enlivened by a sharp-witted script and a fast-paced plot.

She complimented how the episode played with complicated themes in a sophisticated manner, commenting on how the idea of a simulated world has been done before, but how Doctor Who took it further.

[15] Scott Collura of IGN gave "Extremis" a rating of 8.6 out of 10, saying the timing of the episode within the series was perfect to elaborate on the secret of the vault, while also following a larger and more complex story.