42

After The Doctor adjusts Martha's phone for "Universal Roaming", they receive a distress signal from the S.S. Pentallian, a human spacecraft that is hurtling towards a nearby star, and the Doctor pilots the TARDIS towards it to help. After they materialise, the Doctor and Martha are separated from the TARDIS due to rising temperatures. They are forced to help the crew, lead by Captain McDonnell, to try to restart the engines to save the ship and themselves with only 42 minutes left before the ship will be destroyed by the nearby star. Martha teams with Riley to work their way through thirty deadlocked doors, secured via means of pop quiz questions, in order to reach the bridge controls while the Doctor helps the engineering team to repair the engines. Martha is able to call her mother, Francine, on present-day Earth in order to get the answers to some of the questions correctly, though she refuses to answer Francine's questions about the Doctor.
The Doctor learns that one of the crew, Korwin, McDonnell's husband, has been infected with something that is causing his body temperature to rise to incredible levels, and attempts to sedate him while they continue the repairs. However, Korwin manages to overcome the sedative and dons a welding helmet, proceeding to kill various crew members, and in one case, infecting another, named Ashton. As Martha and Riley continue to work through the doors, they encounter Ashton and take shelter in a nearby escape pod, which Ashton is able to launch despite Riley's attempts to stop him. As the pod falls towards the star, Martha calls her mother and apologises, but hangs up when Francine, guided by an official-looking woman listening on the conversation, probes Martha for more information on the Doctor. The Doctor learns of Martha's plight, and puts on a spacesuit to go outside the airlock and activate a magnetic pull control to recover the pod. As he returns into the ship, he looks directly into the star and becomes infected himself, while learning that the star is a living being. As the crew try to help the Doctor, McDonnell admits they drew matter from the star to use as fuel; the living being is fighting back to recover the parts it lost. Martha attempts to help the Doctor into a stasis chamber to prevent him from regenerating, while the crew continues to work towards the front of the ship. However, Korwin appears, and disables the chamber; the Doctor insists Martha to leave him and warn the crew to dump the fuel, which will allow them to escape, while the Doctor struggles to fight the star being from taking over his body.
Martha races through the ship just as the crew reach the bridge, and relays the Doctor's message. Meanwhile, McDonnell has encountered Korwin and, after apologising to him and the rest of her crew for her actions, ejects both of them out of an airlock. As the ship vents its fuel, the engines are able to be restarted and it pulls away from the star; the presence in the Doctor slowly dissipates as the ship moves away. The Doctor, Martha, and the remaining crew regroup and verify they will be safe and that the crew has send a request for more fuel so they can continue their mission. The Doctor and Martha depart in the TARDIS, the Doctor giving Martha one of the TARDIS keys. Martha places another call to her mother, and learning that it is Election Day, offers to meet her for tea. After Martha's call, the woman monitoring Francine's phone confiscates it and leaves, thanking her for her help and for her vote on behalf of Mr Saxon.