42 was the seventh episode of the third series of Doctor Who. It marked Martha’s first trip as an official companion to the Doctor as her previous travels were part of the Doctor’s thanks for helping him in Smith and Jones. It also showed that Harold Saxon was interested in the Doctor and was using Mrs Jones to try locating him through Martha.
In the TARDIS, the Tenth Doctor adjusts Martha’s mobile phone, enabling it to call anywhere in time and space — an ability he refers to as Universal Roaming – as a “frequent flyer’s privilege”. As she is about to telephone her mother, Francine, they materialize on a very hot spaceship in answer to a distress signal. The Doctor notes that the spaceship’s engines are not operating. They open the door to the next room and are pulled through by three members of the crew, who then slam the door shut. The captain, Kath McDonnell, explains that the engines have cut out and left the ship, the SS Pentallian, on a crash course with a local star. A nearby monitor announces that the projected time until impact is 42 minutes. The Doctor suggests evacuating the crew aboard the TARDIS, but the ship has begun venting excess heat through the room in which the TARDIS materialized, rendering it unreachable.
Meanwhile, the others move to the main engine room to try to fix the engines. The Doctor finds that all the engine-related machinery has been destroyed, and comments that the method suggests “someone knew what they were doing.” He observes that they’re in the Torajii system and that they’re using energy scoops for fusion. He thought the scoops might have been outlawed by now; Kath assured him they were due for an upgrade. The Doctor organizes Martha and Riley Vashtee to open a series of password-protected doors in order to access the control room where the auxiliary engines can be activated. He tells Martha to be careful.
The Ship’s medical attendant, Abi Lerner calls the main engine room to say that Hal Korwin, the captain’s husband, is having some sort of seizure. The Doctor tells everyone else to continue trying to save the ship, but Kath goes with him to the sickbay where they find Abi, Ashton, and Korwin. Korwin is lying near a stasis chamber with his eyes closed, screaming in agony, crying, “It’s burning me!” before the Doctor sedates him. Once Korwin is unconscious, the Doctor instructs Abi to test Korwin to find out what is wrong with him; then the Doctor and McDonnell return to the rest of the crew.
While Abi is asked of Korwin’s status, she states he’s heavily sedated and she needs more time, but she doesn’t notice his hands twitching. When she returns to report some very strange results, she breaks off and the crew hears her screams for assistance as Korwin gets up and backs Abi against the wall, saying in a deep voice, “Burn with me”. As he opens his eyes, a blinding light comes out and Abi screams in terror.
While the Doctor runs to Abi’s aid, Martha and Riley Vashtee are opening doors by answering pop quiz-style questions. Riley says they were set by the crew, years previously, after a night of drinking; but when the next question is to find the next in a series of what turn out to be happy prime numbers, the Doctor pops up on the comm with the answer. Riley observes that the crew has changed since they set the questions. The next question category is Classical Music, or, to Martha, 20th century Earth pop culture. The Doctor is both busy and not sure, so Martha rings her mother for help — unaware that Francine has knowingly had her call tapped by a young woman who is presumably a government agent. Meanwhile, the Doctor finds the imprint of Abi and concludes that she was instantly vaporized. He reads the medical reports and reasons that Korwin has been infected in some way, and can now vaporize people.
McDonnell is at first unwilling to believe that Korwin could be responsible for sabotaging the ship and killing Abi, but then relents and alerts the rest of the crew to avoid him. Dev Ashton, working on the engines, sends Erina Lessak a message asking for more tools. She mutes the intercom and mutters under her breath about the injustice of being sent on every errand as she goes to the control cupboard. She sarcastically ends her spiel with “Just kill me now.” When Erina closes the door, she turns to find a helmeted Korwin standing there. He then backs Erina against the wall as he did with Abi, and opens his visor to vaporize her.
Kath asks for the Doctor not to lie about Korwin to spare her feelings. He tells her Korwin is too far gone, then demands to know if there is anything she is hiding. He thinks the tracks are provoked and personal. She says she knows everything about the ship and crew, and no one is hiding anything from her.
Next, Korwin then goes to find Ashton, saying, “They are getting too far” and “we must share the light”. Korwin grabs him around the head, but instead of killing him, Korwin holds his gloved hands to Ashton’s head, which begins to smoke. A helmeted Ashton goes to Martha and Riley — presumably the ones who were “getting too far” — and tells them to “burn with me”. He reaches for his visor, and they run for an escape capsule in terror and lock themselves inside. The capsule begins to jettison, and Martha calls for the Doctor.
Ashton tries to override the system and send Martha and Riley plummeting towards the sun, but Riley is trying equally hard inside the capsule to stop this from happening. Ashton finally just destroys the system when the Doctor arrives and tries to taunt him into raising the visor. At the same time, Korwin finds his wife, telling her, “It’s your fault”, before he is frozen to death by Scannell, prompting the possessed Ashton to ignore the Doctor and seek to continue Korwin’s work. The Doctor calls them, explaining what’s happening, but can’t stop the pod; Martha and Riley jettison as she murmurs, “Sorry.” He calls Scannell and demands a space suit.
In the capsule, Martha implores Riley to have faith in the Doctor, wondering why he has not found anyone in his life to have faith in — his family is all but gone and he has no romantic attachments. However, she is surprised when, in answer to her question, he looks directly at her and says, “I already have.” Resigned to her fate, Martha phones Francine once more and, unwilling to divulge her predicament, instead tells her mother that she loves her and tries to get her to simply converse about her life, until Francine’s probing of whether the Doctor is with her causes a tearful Martha to end the call. Riley comforts her.
Scannell tries to talk him out of it, but the Doctor puts on his suit and goes outside to magnetize the ship in order to pull the escape pod back from the sun. Elsewhere in the ship, McDonnell lures Ashton into the medical bay and freezes him to death in the stasis chamber. Outside, the Doctor struggles to press the magnetic pull control buttons on the side of the ship, and he eventually manages it.
Climbing back into the ship, the Doctor looks at the sun and stares into it, realizing that “it’s alive”, before he too is infected by the same entity as Korwin. The escape pod returns to its launching point. Martha and Riley come back to the ship grinning until they see the Doctor writhing in pain. He cries for them to stay away, and they stand back in shock as his eyes glow before he tightly closes them. McDonnell arrives and the desperate Doctor angrily explains to her that because she illegally mined the sun for fuel, without checking for life signs, she has seriously injured the sun — scooped out the heart of a living being. Kath protests that he can’t know, but he shouts that the sun is alive in him. She gasps and says checking takes too long and they would have been caught using the illegal fusion scoop. With his eyes still shut, he asks the two women to place him into the cryogenic stasis machine to take him below minus 200 to kill the sun entity in him before the entity uses him to kill them.
Korwin’s body begins to twitch. As Riley runs in, Scannell asks his favorite color for the password, then shouts in protest when Riley isn’t sure of the answer.
The women carry the Doctor to the med center. Martha momentarily lets go to open the instructions, and he reaches out and calls for her in panic. She reassures him by repeating his instructions as Kath argues that he can’t possibly survive the temperatures. Martha tells her he’s an alien then shames her away, saying she’s done enough damage. The Doctor frantically tells Martha he can only take ten seconds in the chamber. He cries in pain, saying it’s burning him up, he can’t control it, and “if we don’t get rid of it, I could kill you. I could kill you all.” He confesses he’s scared and tries to tell Martha about a process which may happen if he dies; Martha assures him that he won’t.
Martha starts the freezing process while the Doctor screams, but it is interrupted by Korwin (a corpse possessed by what remains of Torajii in him), who turns off the power to the stasis chamber from the engineering department. The temperature had only reached negative seventy, as the somewhat frosty Doctor yells and continues to struggle with the invading entity. He then tells Martha that she must go to the front of the ship and jettison the fuel, which will return the living particles back to the sun. Martha protests, as she doesn’t want to leave him, but she runs to tell the rest of the crew to jettison the fuel.
Elsewhere, a shocked McDonnell encounters Korwin. She admits to Korwin that this was all her fault and lures him to an airlock. She apologizes to the rest of the crew through her radio, then opens the airlock and tells him that she loves him as the two of them are sucked out into space. Riley and Scannell work on the last door open, while Martha runs towards them. The defrosted Doctor collapses onto the floor and crawls into the corridor. He calls to Martha on the communication system to say he can’t fight it, “give it back or…” He opens glowing eyes as he finishes, “Burn with me.” He screams. Martha reaches the remaining crew as they realize the auxiliaries aren’t working, and tells them to vent the fuel.
As the fuel vents and replenishes the sun, the Doctor’s eyes clear and the auxiliaries fire on reserves, freeing the ship from the gravitational pull of the living sun. The two remaining crew members hug, and Martha runs to find the Doctor and they too hug, both laughing in relief.
The Doctor and Martha head back to the vent room. Scannell and Riley marvel over the size of the unmarred TARDIS, and the Doctor advises they tell the authorities the truth, as the living sun needs care and protection. Martha kisses Riley goodbye. Inside the TARDIS, she tries to joke with a stoic Doctor, who stands there uncomfortably until she asks how he’s doing, then abruptly breaks the moment with a suggestion – ice skating. Martha rolls her eyes and seems a bit hurt as she tells him she’ll go where ever he’d like, then gasps as he gives Martha her very own key to the TARDIS, another “frequent flyer’s privilege”. The Doctor thanks Martha and smirks a little.
Martha calls her mother back, who invites her over for tea, and when asked informs her that it is Election Day. Martha accepts, saying she’ll be around for tea, roughly. Francine hangs up, and the woman and two other men appear to be tapping Francine’s phone again. Confiscating the phone, the woman asks Francine if she has voted. She says she has, but doesn’t say who she has voted for. The woman thanks her for all she has been doing, saying, “Mr Saxon will be very grateful.”