Development Plot and cast details were deliberately kept secret in the build-up to the episode, leading to speculation it would feature cameo appearances of previous incarnations of the Doctor, with Davies later saying that he had conflicting plans for the episode and originally considered including an appearance by the First Doctor and a robot-filled cellar. Before the episode was broadcast, director
Tom Kingsley made a statement attempting to calm speculation, and afterwards Davies explained that the secrecy was because it was "the simplest of the lot", as he had scrapped all other concepts in order to focus on the core premise of Tennant and Tate's characters being alone with the Not-Things on the spaceship. The episode primarily uses VFX and
computer-generated imagery to create the appearance of the spaceship's interior; Davies stated that this was inspired by the 1978 serial
Underworld, which also used special VFX techniques to portray the episode's setting. The episode references
The Timeless Child and the
Flux from his predecessor
Chris Chibnall's era of the show, with the Not-Things taunting the Doctor about events that happened during these episodes. In one scene, the Doctor invokes a
superstition by laying a line of salt to halt the Not-Things. In the following episode, "
The Giggle", this is revealed to be the cause of the appearance of
the Toymaker, and has larger repercussions in
the following season. The VFX and Art teams collaborated on design for the sets, primarily the main hallway. Pre-visualisations were made in order to create references for scene outlines. The design of the ship used in concept art for the episode remained consistent throughout production, with Davies describing the goal as aiming for an "ideal" with the design. Tennant and Tate used physical props to simulate the inhuman aspects of the Not-Things, such as plastic teeth and an oversized controllable arm. Tennant stars as the
Fourteenth Doctor while Tate reprises her role as
Donna Noble. Additionally,
Nathaniel Curtis appears as
Isaac Newton. This episode also marked the first appearance of
Susan Twist, who would reappear in other roles throughout
the subsequent series. George Cheetham acted as a stunt double for David Tennant, with floor runner Helen Langford standing in for Catherine Tate. Additional doubles included Daniel Tuite, Ophir Raray, and Tommaso de Vincenzo for Tennant, and Helen Cripps for Tate. Contortionist Tommaso Di Vincenzo portrayed the Not-Thing mimicking the Doctor in a scene where the creature bent double backwards. Davies had wanted Cribbins to appear in more scenes in the anniversary specials, but due to Cribbins' health he was only able to record one.
Filming The episode was directed by Tom Kingsley. The episode was filmed in July 2022, during a heat wave. == Broadcast and reception ==