The possessed Ood advance and Mr. Jefferson opens fire. On the bridge, Zachary manages to stabilize the system. Rose tries to call the Doctor while Danny gets through the hatch and warns Jefferson that the Ood are using their translation spheres as weapons. Jefferson doesn't believe him and opens the door, and finds Ood on the other side. They kill the remaining crewman and Jefferson opens fire.
Zachary leaves the bridge and tries to escape, only to discover more Ood advancing on him. He retreats to the bridge and seals the door behind him, and then contacts Jefferson on the radio. Jefferson warns him that he's low on ammunition, and Zachary admits he only has a bolt gun with one bolt. the security chief recommends Strategy Nine. Zachary agrees and orders everyone together. Rose admits that she can't get a reply from the Doctor and Ida. The Doctor finally contacts her and explains that the seal has opened, revealing a huge pit but no sign of the Beast. Zachary orders Ida to withdraw, warning that when it opened, the planet almost fell into the black hole. Ida cuts him off and asks the Doctor's advice, and he remembers the Beast saying it was "the temptation." He figures that they opened the prison, but not the cell, and Ida says they should go down. The Doctor considers and actually decides to refuse indulging his curiosity and retreats.
Jefferson prepares to kill Toby, insisting that he's infected. However, Rose intervenes and Jefferson reluctantly but warns that he'll shoot on the first sign of trouble. Toby says that whatever possessed was filled with rage and death, and insists it was the Devil. he embraces Rose, seeking comfort.
Ida explains that Strategy Nine involves opening the airlocks and voiding the atmosphere, while the crew takes refuge. Jefferson activates the elevator but the power shots down. The Ood come on the screen and the Beast speaks through them, warning that they're doomed and only the darkness remains. Unimpressed, Zachary demands identification but the Beast says they know its name, and warns that they will all die there. The Doctor demands to know which Beast it is, given all of the religions that have spread through the universe. The Beast says that it's the truth behind all the myths, and it knows the Doctor as the killers of his light. It explains that the Disciples of the Light chained it in the pit, before the beginning of time. The Doctor doesn't believe it's possible, but the Beast insists and warns that they are all ignorant. It notes the flaws in each of them, and the fact that Rose will die far from home. The Doctor tells her to ignore it, and its image briefly transform into a devil before fading.
The crew begins to panic and the Doctor finally interrupts, telling them that the Beast is playing on their fears. He tells them that they are able to overcome their own fears and are amazing for exploring the universe in their tiny ship. As he tries to rally them, the cable supporting the elevator suddenly snaps. Ida and the Doctor jump clear just in time as ten miles of cable plummet down. Zachary confirms that he has life signs but they've lost comms, and the Doctor and Ida are trapped. They only have 55 minutes of air,
As Jefferson warns Rose they don't have ten more miles of cable, the Ood start breaking through the door. Zachary discovers that the Ood outside his door are cutting through the door belts. They only have ten minutes until the Ood break through. Rose realizes that the Beast cut the Doctor off because he was reminding them to think. She tells Zachary to press the right buttons, and he realizes the rocket has an independent power supply. He has Jefferson open the bypass conduits, and then channels the rocket feed to restore power. They need 100% power for Strategy Nine. Rose turns to Toby and prods him into translating the language from the text. However, he admits he can understand the language better since he was possessed, and goes to work. Next, Rose has Danny work on a way to stop the Ood.
Ida suggests they use the cable to descend into the pit, since it's the only thing they can do before they run out of air. The Doctor insists that he should go down.
Danny comes up with the idea of using a broadcast flare to disrupt the Ood telepathy and create a brainstorm. However, he needs to transmit from the central monitor in Ood Habitation. Jefferson maps out the maintenance tunnels and routes a path, but warns that they're not equipped with oxygen. Zachary informs them that he can manipulate the oxygen field and provide them with a path.
The Doctor jumps off into the pit and finds that there are no walls below. Ida plays out the cable and the Doctor descends.
Danny downloads the broadcast flare program and they enter the maintenance shafts, Jefferson bringing up the rear. Zachary guides them through the tunnels and feeds them air. As they pause for Zachary to oxygenate the next section, they hear noises behind them and Zachary warns them that there are Ood coming after them. The captain opens the gate and directs them to the next section. Jefferson stays behind to cover their escape and opens fire while slowly retreating. The others get to the next panel and Zachary aerates the next section. He can't open it until Jefferson gets past the previous junction. The hatch opens and the one behind them goes down, cutting out Jefferson before he can make it. Out of ammo, the security chief accepts his fate and compliments Zachary on his captaincy. He then asks Zachary to speed up the removal of the air rather than die at the hands of the Ood. Zachary reluctantly agrees and evacuates all the air, and then records his death in the log.
The trio opens the last hatch, but discovers more Ood on the other side. Zachary seals the hatch and Rose climbs up through the hatch above. Danny gets up and the Ood move in on Toby. His eyes glow red, and he gestures them to retreat. He then makes it up. More Ood come after the trio and they flee into Ood Habitation. As the Ood close in on the trio and break through the door to the bridge, Danny uploads the broadcast flare program. When it activates, the Ood collapse in agony.
As the Doctor descends, he talks about the recurrence of the Horned Beast throughout cultures across the universe, and suggests the idea all came from a common source. The cable runs out, leaving the Doctor dangling in the middle of a void with no sensor readings to indicate how much further he has. Ida starts to bring him back up but the Doctor stops the winch and says he has no choice but to drop. He releases his harness and prepares to drop.
At the shaft room, Zachary meets with the others and tries to reestablish contact with the Doctor. Meanwhile, the Doctor discusses faith with her, and insists that the idea that there was something before the universe doesn't fit his beliefs. He thanks Ida and starts to tell her what to tell Rose, and then realizes that Rose already knows. He then finishes unfastening his harness and drops into the pit. Rose gets through on the radio and Ida tells her what happened. Zachary admits to Ida that there's no way to rescue her and he's going to abandon the base. She assures him that it's all right and wishes them luck. Zachary signs off and tells them to get to the rocket, but Rose insists on staying to wait for the Doctor. When she refuse to abandon the Doctor, Toby and Danny hold her while Zachary sedates her and carries her to the rocket with the others. As they get to the rocket, the brainflare effect fades and the Ood start to revive.
The Doctor wakes up and discovers that he is at the bottom of the shaft, and his visor shattered. However, there's a breathable atmosphere and an air cushion that broke his fall. He tries to contact Ida but the communications are jammed.
As Rose starts to wake up, Zachary launches the rocket. They lift off and start to fly away from the planet. The Doctor and Ida hear them depart. On the ship, Rose grabs the bolt gun and demands that Zachary take her back. He doesn't believe her, noting that isn't what the Doctor would want. She relents and Zachary apologizes, warning that it's too late to turn back.
The Doctor finds pictures on the wall showing a battle between the disciples of light and the Beast. He finds a series of glowing urns and realizes that they are the bars of the prison holding the Beast. The creature appears before him.
On the ship, Toby starts chuckling. He claims that he's simply relieved, and confirms that the gravity well is holding.
The Doctor realizes that he was expected and the Beast needs him for some reason. It refuses to answer him and the Doctor realizes that it can't talk now, but it spoke earlier. The Beast can't speak now because it has projected its consciousness elsewhere. If the prison is opened, the gravity field collapses and the planet is destroyed. To avoid that fate, the Beast sent its mind out and vows to stop it. He picks up a rocket and starts to destroy the urns, but then realizes that the Beast has set it up so that the Doctor will have to sacrifice Rose.
On the rocket, Rose realizes that the Beast could have killed them at any time, but it let them go. Toby tells her to shut up and says that they will be beyond the reach of the black hole in 40 seconds.
The Doctor insists that he believes in one thing: Rose. he smashes the urns.
The gravity field collapses and the rocket plummets toward the black hole.
The Doctor shouts that the Beast will fall into the black hole, and he'll go down with it.
Rose realizes that the planet is falling into the black hole. She turns and sees Toby, his face covered in runes as the Beast manifests, insisting it will never die. Rose grabs the bolt gun and shoots the window. She then unfastens Toby's belt and he's pulled out into the vacuum and falls into the black hole. Zachary establishes the emergency shields, but warns that they can't save themselves.
In the pit, the Beast's body is consumed by flame.
Ida collapses as her air runs out.
The Doctor runs from the falling debris and finds the TARDIS.
Aboard the rocket, Rose, Zachary, and Danny watches as the planet falls into the black hole. The rocket starts to fall after it... and then they stop. The ship turns around and the Doctor calls to inform them that he's projected a tractor beam from the TARDIS and is towing them to safety. He jokingly offers Ida in return for Rose, but admits that he couldn't save the Ood. Rose enters the TARDIS and they embrace. Aboard the rocket, Ida admits that she doesn't remember what happened and Danny tries to explain the TARDIS without success. The Doctor wishes them safe travel and admits that he doesn't know what Beast was. When Rose wondered what it meant about her dying in battle, the Doctor tells her that the Beast lied. He tells the crew that he and Rose are the stuff of legend and dematerializes. The crew departs as Zachary files his final log report.