As they travel in the TARDIS, the Doctor is adjusting Martha's cell phone to give her "universal roaming." She starts to make a call but the TARDIS picks up a distress signal and the Doctor makes an unsteady landing on its source: the cargo ship SS Pentallian
. They've materialized in a venting chamber and the room is sweltering. The duo go through a door to the next chamber where the ship's crew come running up to them. They quickly close the door and say that the engines shut down four minutes ago. The ship goes into safety lockdown and a display says they have 42 minutes until collision. Martha looks out a porthole and sees that the Pentallian
is falling a sun.
There are seven crew members on board: Captain McDonnell, her husband Korwin, Ashton, Riley, Orin, Abi, and Erina. The Doctor offers to evacuate them and opens the venting chamber, only to discover it's now filled with superheated steam. With access to the TARDIS cut off, the Doctor goes to Engineering to make repairs. The systems have been thoroughly sabotaged and it's clear someone who knew what they were doing was at work. They can activate the emergency generators from the bridge, but it's at the front of the Pentallian
and there are 29 deadlock-sealed doors between them. Riley is familiar with the systems and offers to start decoding them, and Martha goes along to assist.
Abi, the medic, calls McDonnell to the infirmary, saying Korwin is having a seizure. The Doctor goes along with McDonnell and they see that Korwin is screaming about being burned. Ashton reports that he saw Korwin sabotaging the engines. The Doctor sedates Korwin and uses a scan to determine he's giving off high-energy heat readings. He says that Korwin is infected and they should put him in the infirmary's stasis chamber to slow the spread of the "disease."
Riley and Martha start bypassing the security doors. Each one was given a different question/password by the crew the last time they were on leave, but they were drunk and not everyone remembers each one. Some of the crew at the time are no longer on the ship, and they only get once chance per door. With the Doctor's help over the intercom, they move through the cargo ship door by door. He also warns Martha that there's something on board that could pose a danger to them. When the next door has a question about "classical" 20th century music, Martha calls her mother via her universal cell phone. At the same time, Abi calls the Doctor and McDonnell to inform them that Korwin's body is mutating. Korwin attacks her and growls "Burn with me!", then opens his eyes and releases a blast of heat. The Doctor and McConnell run to investigate while Francine, hearing Abi's scream over the intercom, asks what's happening. Martha quickly hangs up on her.
Korwin dons a helmet and leaves the infirmary. The others arrive a few seconds later and discover nothing left of Abi but a vaporization shadow on the wall. The Doctor examines Korwin's test results and determines that he is no longer human. He asks McDonnell what kind of contaminant they might have brought on board but she insists there's nothing that could cause this kind of infection. She warns the crew to be careful and Ashton sends Erina, his assistant, to get some parts. As she complains, Korwin attacks her, opening the visor of his helmet and releasing another heat blast.
McDonnell demands to know what the Doctor thinks of her husband's chance of survival, and wants him to tell her the complete truth. He emotionally notes it's too late for Korwin and again asks what the crew might have done to set off the infection.
Riley and Martha are working their way forward when Korwin goes after Ashton. Korwin says that they're getting too far and grabs Ashton's head. Extreme heat interrupts as Korwin passes on the "infection." The possessed Ashton dons a helmet and goes after Riley and Martha. Martha gets herself and Riley through a door only to discover they're in an escape pod and the automatic jettison sequence has activated. Riley attempts to abort the sequence but Ashton tries to stop him using the outside controls. Martha calls the Doctor for help and he come running. McDonnell and Korwin go to Engineering only to discover that the generators have been sabotaged. Korwin moves toward them, saying that it's McDonnell's fault.
The Doctor confronts Ashton and demands to know what he wants. Riley succeeds in stopping the launch system but Ashton smashes the controls. The Doctor, angry, marches up to Ashton and demands to see who he's dealing with. Ashton starts to open the helmet visor. McDonnell tries to get through to Korwin, who seems to momentarily recognize her. He prepares to kill her but Orin unleashes coolant, freezing Korwin solid. Ashton senses the attack and walks past the Doctor heading for Engineering. The Doctor runs to the portal and calls out to the departing Martha that he'll save her no matter what.
McDonnell is furious at Orin for killing her husband, but Orin notes that the Doctor had said Korwin was already too far gone. She doesn't know if she should believe the Doctor but Orin says they need her focusing on the problem at hand. She goes to deal with Ashton while Orin helps the Doctor find a spacesuit. In the escape pod falling into the sun, Riley admits he's never really anyone or anything stable in his life, and Martha insists the Doctor will keep his word and save them. However, she realizes that she may never see her family again.
McDonnell lures Ashton into the infirmary and manages to get him into the stasis chamber. She activates it, freezing him solid.
The Doctor dons the spacesuit and prepares to go outside to activate the magnetic tractor beam to pull the escape pod back to the Pentallian
. He tells Orin to continue working through the security doors.
Martha calls Francine to have a normal conversation and says that she loves her. On Earth, a blonde woman is in Francine's house monitoring the call, and tells a cooperative Francine to keep the conversation going. Francine tries to get Martha to talk about the Doctor but Martha hangs up and hugs Riley.
The Doctor climbs out the airlock and tries to reach for the magnetic controls. He manages to remove the cover and activate the tractor beam, pulling the escape pod out of the sun's gravity. The Doctor looks down into the sun and hears a growling noise and sees flares of energy, and realizes that it's a living creature. he and Martha are reunited but his eyes are glowing with heat energy. Angry, the Doctor says that McDonnell and the crew are responsible: they used illegal fusion scoops to mine the sun's heart for fuel without checking for life signs. The sun itself is alive and trying to recover its stolen heart by pulling the ship into itself. The Doctor warns that they closer they get to the sun, the more influence the star creature will have over him. He tells her to get him to the stasis chamber and freeze him before he's forced to kill everyone.
In Engineering, Korwin starts to thaw and gets up. Riley goes to help Orin get through the security doors, while Martha and McDonnell get the Doctor to the infirmary. The Doctor, frightened, begs her to help and tries to explain that he may regenerate. She promises him he'll be okay and starts to activate the stasis chamber. Korwin, seeing the display in Engineering, shuts down the power to the stasis chamber. Realizing who's responsible, McDonnell heads to Engineering. The Doctor tells Martha to go to the bridge and vent the fuel back into the sun, and she reluctantly agrees.
McDonnell lures Korwin out of Engineering and hides in an airlock. He quickly realizes where she is and enters after her. She backs up against the far wall and apologizes for harming the star creature. As Korwin grabs her and prepares to open his helmet, she hits the airlock cycling button and the door opens, jettisoning the two of them out into the sun.
The Doctor staggers out of the infirmary and contacts Martha on the intercom, saying he can't fight it any more. His eyes glow red hot and he says Martha will burn with him. Martha gets to the bridge as Riley and Orin open the final door. She tells them to vent the fuel and it drifts down to the sun. The men activate the generators and the star creature releases its "grip." The ship pulls away from the sun with seconds to spare, while Martha runs back to the now-recovered doctor and they hug.
Later, Riley and Orin have recovered the TARDIS and say their farewells. Riley says he's finally found something to believe in and kisses Martha, who kisses him back. She enters the TARDIS and the Doctor gives her a key, and his thanks for saving him. Remembering her mother, Martha calls Francine who suggests that she come around for dinner that night. Martha asks what day it is and Francine says it's Election Day. As Martha rings off, Francine gives her phone to the blonde woman and her two operatives. They talk briefly about the election and the blonde woman says that Mr. Saxon will be very grateful to Francine for her help.