The Doctor (David Tennant) and Martha (Freema Agyeman) receive a distress call from the S.S. Pentallian, a human ship which is heading for the sun in the Torajii system. The Doctor lands on the ship with the TARDIS, but once he and Martha leave, they can’t get back to it because of the heat. They only have 42 minutes before the ship falls into the sun. There are thirty deadlock doors between them and the bridge, all password encoded. Martha teams up with Riley Vashtee (William Ash) to answer pub-quiz questions to open doors, while the Doctor helps the engineering team to repair the engines. Martha calls her mother (Adjoa Andoh) to get the answer to one question.
Captain Kath McDonnell’s (Michelle Collins) husband Korwin (Matthew Chambers) has been infected and his body temperature is rising. They try to sedate him, but he runs. He puts on a welding helmet and starts killing crewmembers. He infects someone named Ashton (Gary Powell). He takes shelter in an escape pod with Martha and Riley, then launches the pod, but McDonnell freezes him.
The Doctor spacewalks to retrieve the pod. He is infected by the sun and learns that the sun is a living being. The crew has removed its heart to use as fuel and it is trying to get it back. Martha puts the Doctor in a stasis chambre to cure the infection, but Korwin disables the chambre. The Doctor tells the crew to dump the fuel. McDonnell seizes Korwin and spaces them both. The fuel vents, the engines start, and they pull away from the sun.
The story is supposed to be 42 minutes long in real time. It takes place in the 42nd Century. And 42 is the answer to the question of life, the universe, and everything in Hitchhikers Guide to the Galaxy, by Doctor Who writer Douglas Adams. This was the first story written by Chris Chibnall, who later became showrunner. Martha gets a phone upgrade and a key to the TARDIS, so she is a real companion, like Rose. The masks worn by the possessed crewmen were inspired by Cyclops of the X-Men. We discover that Martha’s mother Francine works for Mister Saxon. Michelle Collins is impressive as Captain McDonnell. Martha is the big damn hero in the story. The directing is remarkable, with its wide shots, low-angel shots, long tracks, and quick cuts, and the fast pace is exhilarating, enhanced by the use of colour and the music of Murray Gold.