The Flux is Coming... (TV story)

The Flux is Coming... was a prelude to Doctor Who: Flux, teasing the titular Flux through an in-universe distress call from the Thirteenth Doctor. The story was also the first appearance of Dan Lewis as a companion, taking place after, but airing before, his coming aboard the TARDIS in the full-length television episode The Halloween Apocalypse. John Bishop's as-yet-unnamed character had earlier been revealed in an especially-shot minisode, Welcome to the TARDIS..., which aired after the credits of Revolution of the Daleks, taking place prior to his meeting the Thirteenth Doctor.

Publisher's summary
Be ready...

Plot
The Thirteenth Doctor sends a distorted distress call while Yaz and Dan studiously pilot the TARDIS at the console. The Doctor warns of the impeding arrival of the Flux as well as Sontarans, Weeping Angels, the Ravagers, and other creatures from across the universe. Dan calls to the Doctor, telling her that they need her help, so the Doctor concludes by saying "be ready".

Cast

 * Thirteenth Doctor - Jodie Whittaker (uncredited)
 * Yasmin Khan - Mandip Gill (uncredited)
 * Dan Lewis - John Bishop (uncredited)

Continuity

 * The Doctor mentions several enemies who appeared in TV: The Halloween Apocalypse.