Part one

A storm rages on the planet Karn. One of the sisters finds the TARDIS buried beneath starship debris from which the Twelfth Doctor emerges requesting to be taken to Ohila.

On prehistoric Earth, the Eleventh Doctor and Alice Obiefune search for a fruit that Jenny may gift Vastra for their anniversary. The fruit Alice finds however is a nanjura fruit which, though delicious to Time Lords, is toxic to Silurians. As the Doctor snacks, a dinosaur breaks through the brush and the two start to run.

In the 24th century, the Tenth Doctor, Gabby Gonzalez and Cindy Wu travel via "space bus" through the Null Zone to Cosmomart, a planet that has been converted into a shopping mall. Despite the warnings of the pilot droid, the three transmat into the mall to find it abandoned before they are ambushed by Sontarans.

In the Powell Estate in the year 2006, Jackie Tyler flees from a mechanised dog before she is rescued by the Ninth Doctor, Rose and Jack Harkness on a flyer. Though Jackie believes the ruined state of London is the Doctor's doing, the planet has been invaded by the Cybermen. However, the Doctor believes there is more going on, as the brutal efficiency of the invasion is beyond even the Cybermen. The group's first order of business is to retrieve the TARDIS, held at the converted St Paul's Cathedral.

At Cosmomart, the Tenth Doctor surrenders to the Sontarans while Cindy lobs a shop dummy head at a soldier. Though she and Gabby are ready to run, the Doctor is adamant that they obey the Sontarans' commands. The three are led by Commander Sturg to the Sontaran flagship where Cindy realizes the Doctor wanted to be captured.

As the Eleventh Doctor and Alice flee, their pursuer is suddenly killed by a tyrannosaurus rex with a cybernetic implant. Before the apex predator can eat them, they are surrounded by Cyber-Silurians using other Cyber-converted dinosaurs as beasts of war.

On Karn, the Twelfth Doctor reveals to the Sisterhood that there are vortex tsunamis occurring throughout the Time Vortex. Though he has tried to reach Gallifrey he cannot breach the transduction barrier nor contact the High Council. As such, he has come to request usage of their "door". The secret door that the Sisterhood use to reach Gallifrey that no one is supposed to know about. Before Ohila can deny him access, Cybermen enter the chamber. With no alternatives, Ohila leads the Doctor to the door and sends him through, closing it to prevent the Cybermen from following. The Doctor runs to the High Council chamber, calling for his fellow Time Lords to break out the heavy artillery to deal with the crisis. Arriving in the Council chamber however, he is greeted by the sight of Rassilon in command of the Cybermen.