The one that is self-aware of the episode runtime.
Written by Chris Chibnall.
The Doctor and Martha land the TARDIS aboard a spaceship, only to have the room they landed in rise to unbearable temperatures, trapping them aboard the ship, which is being pulled into a nearby sun. Martha and the Doctor have only forty-two minutes to save the ship, the crew and themselves. Meanwhile, a member of the crew has contracted a mysterious illness. His body temperature rises to an impossible height just before he opens his burning-white eyes and absolutely fries a fellow crew member to a scorch mark on the wall.
The only way to save the ship is to pass through a security system, which the crew inconveniently programmed while they were drunk, and decided that it would be a good idea to make it a series of obscure trivia questions. It's like the Pub Quiz from Hell. Martha phones her mum, her own Voice with an Internet Connection, and gets some answers... unaware that Mrs. Jones' phone is being tapped by the mysterious people from "The Lazarus Experiment", the ones with sinister music cues; they are revealed to be government agents working for Mr. Saxon.
Before Martha and the other guy make it to the finish, though, another crew member gets infected with the mysterious burning illness and corners Martha and another crew member in an escape pod. The Doctor confronts the infected crew member who passes him by and leaves the area. The Doctor rushes to the escape pod controls but it is too late, the escape pod has already been released. Martha and the crewman are falling into the sun, and the escape pod retrieval controls are on the outside of the ship... Fat lot of good they'll do there.
The Doctor dons a familiar orange spacesuit and saves Martha's ass, but as he's clinging to the side of the spaceship he gets a very clear view of the sun and the sun gets a very clear view of him. Apparently the sun is alive, and it's angry. The Doctor, succumbing to the same possession by the sun that claimed the other two crew members, reveals that the only way the sun will let them go is if they return the stolen sun particles the ship has been using for fuel. Martha takes care of it and saves the day, because the Doctor's a bit preoccupied with going batshit insane.
- Chekhov's Gun: The mention near the beginning that the ship is running off illegal fusion scoop technology.
- Continuity Nod:
- As he's being put into stasis, the Doctor tries to tell Martha what happens to Time Lords who are near death, having obviously learnt from the confusion Rose (as well as Ben, Polly, the Brigadier, Sarah Jane, Tegan, Nyssa, Adric, Peri, Mel and Grace) went through way back when.
- The Doctor once again exhibits his ability to withstand extremely cold temperatures, as he tells the crew to put him in the stasis pod and set it to below -200°C when he's possessed by the sun, telling them he'll survive.
- The Doctor finally gives Martha the Super Cell Reception upgrade and a key to the TARDIS.
- Convection Schmonvection: As per "The End of the World", the one thing that stops everyone aboard from getting fried is heat shields.
- Demonic Possession: What the sun does to Korwin, Ashton and the Doctor.
- Dwindling Party: The ship starts off with seven crew members (McDonnell, Korwin, Ashton, Abi, Erina, Riley and Scannell). Of those seven, only two (Riley and Scannell) are alive at the end of the episode.
- Dying Declaration of Love: McDonnell tells Korwin she loves him, then embraces him before the two of them are sucked out of an airlock into space.
- Eldritch Abomination: The antagonist is a living star that can possess people and give them burning eyes.
- Famous Last Words: "Please, kill me now." (courtesy of Erina)
- Fantastic Aesop: Remember kids, always check to make sure that the star you're using as a fuel source isn't sentient...
- Fireball Eyeballs: Those possessed by the sun appear to have literal stars for eyes, bright enough to kill.
- Mr. Saxon sent people to listen to Martha's mother's phone.
- Also marks the first appearance of the Master's Leitmotif and the drumbeat.
- Genius Loci: The sun is pissed off about the solar scooping.
- Glowing Eyes of Doom: The signature defining characteristic and weapon of those possessed by the sun. Used with terrifying effectiveness when the Doctor reveals his glowing eyes just before he forces them shut as the sun begins to possess him.
- Heroic BSoD: After all's said and done, Martha finds the Doctor staring into space.
- "I Know You're in There Somewhere" Fight: The Captain tries this with her (now deceased) husband. It works for a little bit long enough for the Captain to jettison herself and her former husband into the sun.
- Internal Homage: A sentient celestial body has its substance stolen for use as fuel. In revenge, it possesses the thieves, giving them Glowing Eyes of Doom, and makes them kill others. It also starts drawing the ship towards it. The only way to avoid a collision is to jettison all the stolen material.
- In the Style of...: The entire episode is in real time, complete with a handy timer describing how much time is left until the ship crashes into the sun, just like a certain suspense show that moves forward in real time and has an ever-present timer.
- I Thought Everyone Could Do That: The Doctor seems genuinely surprised and annoyed that nobody else can mentally calculate the next happy prime.
- It's All My Fault: McDonnell, after running into the possessed Korwin for the last time, admits that mining the sun for cheap fuel was indeed her fault, but that she honestly didn't know the sun was alive.
- Just in Time: The ship's impact with the sun is averted with two seconds left.
- Karma Houdini: The team do not defeat the sun, they merely give it what it demands. True, the crew took part of its substance first, but they had no idea it was alive.
- Kill It with Ice:
- Captain McDonnell kills the sun-possessed Ashton by shoving him in the stasis pod and setting it to -273°C otherwise known as absolute zero, the coldest temperature it's possible to physically reach.
- The Doctor tells Martha to freeze him to free him from the sun's influence. It doesn't work because the possessed Korwin shuts down the power to the stasis pod.
- Last-Name Basis: McDonnell, Korwin, Ashton and Scannell are primarily referred to by their surnames. Their respective first names are Kath, Hal, Dev and Orin.
- Leitmotif: The aforementioned "The Master Vainglorious", which foreshadows the oncoming Saxon revelation. Also "All the Strange, Strange Creatures", the main action theme of Series 3, culminates in this episode; seeing as how it's one of the most dangerous adventures of the season. The synth beat and howling choir simply own the last third of the episode.
- Light Is Not Good: A vengeful sentient star trying to kill everyone on the ship.
- Meaningful Name: At least part of the device used to open the locked doors is appropriately branded "Icarus".
- Men Are the Expendable Gender: Inverted. Only male crew members survive the episode.
- Mind Rape: In what becomes a trend for the Tenth Doctor, he gets possessed by an angry living star.
- Motor Mouth: The Doctor, during his explanation of happy primes, rattles off a rather long sentence without stopping for breath at all."Any number that reduces to one when you take the sum of the square of its digits and continue iterating it until it yields 1 is a happy number, any number that doesn't, isn't. A happy prime is both happy and prime. Now type it in!"
- Oh, Crap!:
- "Burn with me..." triggers this in everyone and it's dialled Up to Eleven near the end with "Burn with me, Martha..."
- The Doctor's facial expression when he's staring at the sun and discovers "It's alive."
- One Password Attempt Ever: The ship's secure closure means that the people unlocking them only get one chance per door. Get one question wrong, and the entire system freezes.
- Only Smart People May Pass: In theory, the pub-quiz style problems are trivia passwords set by the crew. Too bad several crew members have either rotated out or been killed, leaving the Doctor and Martha to figure out most of the answers.
- O.O.C. Is Serious Business: This is one of the rare occasions where the Doctor ever says that he's scared.
- Orange/Blue Contrast: During the scenes where Martha and Riley are stuck in the escape pod, which has very blue lighting to contrast with the orange light from the sun. Particularly given that orange is this episode's predominant colour.
- Race Against the Clock: 42 minutes until they crash into the sun.
- Real Time: "42" is 42 minutes long.
- Redemption Equals Death: McDonnell atones for stealing the sun's energy by tossing herself and her infected husband into the sun.
- Running Time in the Title: 42 minutes, not counting the title sequence and end credits.
- 42, in honour of the late Classic Who writer/producer Douglas Adams.
- It's also 24 flipped around, which is a more relevant reference (see In the Style of... above).
- After Martha asks the Doctor about Elvis and the Beatles, he says "Here comes the sun", which is a song by the Beatles. Suspiciously appropriate.
- Shown Their Work: Happy Primes exist.
- Sinister Sentient Sun: The sun of the Torajii system, which has a mind of its own and tries to pull a spaceship into it and destroy it. Subverted, as it doesn't try to destroy the spaceship for malicious purposes, but because its crew had stolen its core, and it wanted it back.
- Taking You with Me: McDonnell ends up jettisoning herself with her possessed husband.
- Tempting Fate: Erina, annoyed with being the one asked to fetch tools, asks aloud for somebody to "Kill [her] now", a request the infected Ashton is more than willing to grant.
- Thrown Out the Airlock: At the end, McDonnell does this deliberately to herself and Korwin, apologising to Riley and Scannell over the intercom, giving a Dying Declaration of Love to Korwin, then embracing him before they are ejecting into space. Riley and Scannell look on in horror when they realise what McDonnell just did.
- Too Dumb to Live: Erina gripes about being asked to fetch tools to fix the ship while it's falling into the sun because of instrument failure.
- Used Future: This spaceship is old, failing and has to resort to an illegal method of fueling itself up.
- Voice with an Internet Connection: Martha calls her mother for help with one of the password questions.
- What the Hell, Hero?: The agonised and semi-possessed Doctor gives one to McDonnell for not checking they were stealing from a lifeform. Martha refuses her help to treat him, snapping coldly that she's done enough.
- Writers Cannot Do Math: Averted with the "What's the next number in the sequence?" puzzle happy primes are a thing, as mentioned in Shown Their Work above. But also invoked, as there are simpler solutions to this puzzle (421 if the differences between the numbers go up by 18 each time and 439 if the differences double each time), and with this kind of puzzle, the right solution should always be the simplest one, or else the puzzle is no longer fair.