Inspirations: 42

As with last week's episode, the inspiration for this story can be seen the title.

In November 2001, the Fox network launched the thriller series 24, whose gimmick was that each instalment would cover one hour in real time - so the overall season of 24 episodes would combine to show 24 hours in the lives of the various protagonists.

This format meant that there was a built-in deadline to events, making it much more exciting for the viewers. The heroes, led by Kiefer Sutherland as counter-terrorism agent Jack Bauer, had to defeat the villains within a strictly limited timescale.

It was decided for Series 3 of Doctor Who to attempt something similar - in the hope that it would capture the same sort of excitement. 

When you removed the opening and closing credits, the average episode of Doctor Who at the time comprised 42 minutes of drama - which just happens to be 24 reversed.

This provided the story with its title.

As it was, the episode was not played in real-time. There is a jump of a couple of minutes at one point.

Writer Chris Chibnall was working on the first series of Torchwood when he was asked if he wanted to write his first Doctor Who episode. Elements including an intelligent sun, glowing eyes and a spacewalk were handed to him by Russell T Davies. 

An early version had a space station setting, in orbit around the star and studying it for generations, but this was changed to a passing ship only recently arrived, to save on a more complicated backstory.

Chibnall was pleased to note that '42' also had great significance for Douglas Adams fans, being the answer to Life, the Universe and Everything.

The spaceship was going to be called the S.S. Icarus - after the mythological figure who was the son of Daedalus, architect of the Labyrinth on Crete. In order to escape the island they decided to make wings for themselves. Daedalus cautioned his son not to fly too close to the Sun as the heat would melt the wax holding the feathers on. Icarus failed to heed dad's advice, and plunged to his death.

A problem arose when the Danny Boyle sci-fi movie Sunshine was released in 2007. 

This had used the intended spaceship name (an Icarus I and an Icarus II), so it had to changed. It became the S.S. Pentallian - a name derived from Revenge of the Cybermen, where a "Pentallian drive" had been an essential component of the transmat.

Sunshine sees a space mission to reignite the dying Sun. (Considering the derivation of the name, it is pushing luck somewhat to name the ship after a failure - especially when the first ship was lost several years before). One of the inspirations for his film cited by Boyle was the Russian movie Solaris (1972) which featured a sentient planet. Ridley Scott's Alien was another influence. Both have elements which can be seen in 42 - a crew trapped on a spaceship with an alien killer in both, and the sentient celestial body in the former. Like 42, Sunshine has a major action set-piece revolving around the ship's airlock.

There is also a hint of the 1997 film Event Horizon, in terms of a spaceship encountering a natural phenomenon (in this case another dimension) which proves to be sentient and which affects a crewman, turning him into a killer.

The masks worn by the infected crew were inspired by Cyclops of X-Men. His eyes fire beams of energy, which he controls behind a dark visor.

The Doctor upgrades Martha's phone so it has universal roaming. He had earlier offered this to Rose in The End of the World.

We see that Francine Jones is allowing people to monitor her daughter's calls. They are employees of Harold Saxon - this series' story arc - and we saw in the previous week's episode Francine falling under their corrupting influence. Then, it had been a man played by Bertie Carvel, but his career suddenly took off and he was unavailable for later episodes and so his place was taken by a blonde female character, played by Elize Du Toit.

The story was going to be set at the same time as The Impossible Planet / The Satan Pit, and at one point was going to have Ood amongst the crew.

Riley's surname was going to be Kinkade, but that name had already been used as middle name for Brannigan in Gridlock.

The star system was going to be called the Peony System, but was changed as it sounded like people were saying "penis".