The story unfolds with the Doctor fleeing from Swarm, carrying Tecteun's Ood, amid a tense confrontation. As she escapes, Swarm and Azure push forward to capture her, but she swiftly removes her conversion plate. In a dramatic moment, when Swarm touches her, she gets split into three copies, each ending up in different locations: the Division spacecraft, Bel's ship, and the Liverpool tunnels.
On the Division spacecraft, Azure tries to restore the Doctor's lost memories using a fob watch, but the Doctor, displaying determination and mistrust, refuses to look. Azure then reveals her sinister plan: to engineer the Flux into a cycle of universal destruction, a catastrophic event to wipe everything out. Meanwhile, the Ood tries to slow down the deadly Flux from spreading further.
In another location called the Passenger, Vinder and Diane work together to disable its internal systems, managing to escape into an unfamiliar environment, seeking safety. Elsewhere, Yaz, Dan, Jericho, and Williamson face off against a group of Sontarans. Using lightning-powered attacks from a tunnel door, they successfully defeat the vanguard and then travel through a portal to 2021. There, they meet Kate Stewart, who is leading a resistance movement fighting against the invasion. The Doctor makes a crucial bargain with Kate’s team, enlisting Claire and Jericho to help pinpoint the final Flux event’s coordinates. She then swiftly takes her TARDIS from Kate’s control.
The Doctor’s pursuit leads her to crash Bel’s ship into the Sontaran command headquarters, but they are stopped by a force field. She is captured and taken aboard a Sontaran ship. During this captivity, Karvanista, a Lupari, reveals that he was once her companion during her time in the Division, though he cannot disclose more because of a dangerous Division poison implant in his brain. The Sontarans learn from their transmissions that the Lupari are extinct.
Determined to free herself, the Doctor pilots her TARDIS to rescue a copy of herself from torture inflicted by the Grand Serpent. She then directs her TARDIS at him in a show of defiance. Meanwhile, Bel uncovers Sontaran transmissions plotting an alliance with the Cybermen and Daleks, setting a trap to sacrifice them to the Flux while the Lupari deploy their shielding to protect the Sontarans. Claire manages to escape her captors, but Jericho remains trapped. The Doctor succeeds in rescuing Vinder and Diane, then uses the Lupari shield to envelop and destroy the Sontarans, the Daleks, and the Cybermen, who are overwhelmed by the Flux. She then employs the Passenger — a vast repository of endless matter — to absorb the destructive power of the Flux itself.
At Atropos, Azure and Swarm attempt to sacrifice the Doctor to Time. However, since the Flux has failed to free Time, it instead destroys Azure and Swarm. Time chooses to let the Doctor go but warns her of her impending end, before ultimately reunifying her with her future self. In the tunnels, Kate Stewart and Vinder manage to maroon the Grand Serpent on a small asteroid using a portal. Vinder and Karvanista decide to travel together, while Dan asks Diane out on a date — though she declines. The Doctor then invites Dan and the others—Yaz and Jericho—to join her on new adventures. As a final gesture, she deposits her fob watch deep inside the TARDIS, hinting at more mysteries to come.
This complex tale weaves themes of sacrifice, trust, and the battle between destruction and hope, set against the backdrop of a universe on the brink of annihilation.
