The Doctor (Jodie Whittaker), Dan (John Bishop), and Yaz (Mandip Gill) are transported to Sevastopol during the Crimean War, where they meet nurse Mary Seacole (Sara Powell) before Yaz and Dan are transported away in Time. The Doctor cannot enter her TARDIS to track them. In Sevastopol, she realizes that the Chinese and Russian troops are all Sontarans. The Doctor then discovers that seconds before the Lupari shielded Earth from the Flux (in the previous episode), the Sontarans used the Flux to rewrite human history so they could enjoy a battle to the death.
Yaz materializes in the Temple of Atropos on the planet Time, where she meets British philanthropist and tunnel-builder Joseph Williamson (Steve Oram) before befriending Vinder (Jacob Anderson). They are recruited to repair the temple’s mainframe. The evil Swarm (Sam Spruell) and Azure (Rochenda Sandall) arrive to take control of the temple’s power. Dan is transported back to Liverpool, where he finds the city under the control of the Sontarans. His parents save him from a Sontaran unit. It seems a three-minute blackout caused by the Lupari shield allowed the Sontarans to land at the Liverpool Docks, where they set up base. Dan sneaks onto one of the Sontaran ships and contacts the Doctor, who asks him to beat the Sontarans.
He is discovered by them and is saved by Karvanista (Craig Els). They ram the captured vessel into the shipyard, evacuating at the last moment, to destroy the primary Sontaran fleet. This resets the timeline. Meanwhile, the Doctor poisons the supply system of the Sontarans in Crimea. This cripples their offense, forcing them to retreat, and the British General Logan (Gerald Kyd) blows up their vessels. The Doctor gets access to the TARDIS to seek out Dan and Yaz.
The TARDIS malfunctions and forces the Doctor and Dan into the temple, through which all temporal energy must pass. The Flux, it seems, was created by the death of the Mouri gatekeepers and the ruin of the temple. Swarm is now trying to control Time for himself. He has imprisoned Yaz and Vinder. The Doctor knows the power unleashed will kill Yaz and begs him not to do it, but he ignores her.
The episode, Chapter Two of the six-part Flux, was written by showrunner Chris Chibnall and directed by Jamie Magnus Stone. It received very positive reviews by critics. Sontaran General Skaak (Jonathan Watson) mentions the Sontaran Linx, who claimed Earth for the Sontaran Empire during the third Doctor’s time. When the Doctor is asked where she is going, she says, “Half a league onward,” quoting The Charge of the Light Brigade by Alfred Lord Tennyson. Dan gains access to the Sontaran ship by using the Vulcan hand-salute from Star Trek.