The TARDIS materializes in what appears to be a closet. It reacts oddly, and the Doctor notes that it's acting queasy. They set out to explore and the Doctor quickly realizes they're in a deep-space sanctuary base. They enter a central area and hear the sound of drilling. Rose sees the words "Welcome to Hell" written on the wall, surrounded by writing in some other language. The Doctor doesn't recognize it and notes that it goes beyond the range of the TARDIS's knowledge. They open another door and are confronted by a series of humanoid creatures with tentacles for mouths. They advance on the Doctor and Rose, saying "We must feed." The creatures close in and then pause, adjust their translation spheres, and say they must feed them if they're hungry. The creatures, the Ood, note the electromagnetics interfere with their speech systems.
The base crew arrive and are curious to know who they are. The security chief, Jefferson, calls in the acting captain, Zackary Cross Flane, and then wonders how they can't know where they are. A quake shakes the best and Jefferson herds the two of them to the sheltered central command area where the rest of the crew are located. As the crew prepares for impact, the Doctor wonders what the name of the planet is and the science officer, Ida Scott, informs them it doesn't have a name. The base is shaken by the impact and part of the surface caves in. Zack sends Toby to check the rocket link. Toby objects but Zack tells him to obey. Ida is still surprised they don't know where they are. She pauses to introduce them to the crew: Danny Bartock, Ethics Committee; Scooti Manista, trainee, and Toby Zed, Archaeology. Zack opens the observation dome to reveal that the planet they are on is in orbit around a black hole. The Doctor notes that it's impossible and nothing can escape it.
As Toby heads to the rocket link, he hears a voice whisper his name. No one responds when he asks who it is and he continues on his way.
The crew assemble and Zack and Ida gives the Doctor what information they have on the black hole. Zack explains that the planet somehow generates a gravity field to keep it in balance against the black hole. The field extends out from the planet as a funnel and they flew down it. When they came in, they lost the captain and Zack was put in charge. The Doctor quickly calculates how much power such a field would take. While he works, an Ood brings Rose a drink. It explains they have no titles and they are as one. She talks to Danny who is surprised she's never heard of them. He describes them as slaves but notes that if the Ood don't have orders to obey, they pine away and die. The Doctor finishes his calculations and notes that the calculations indicate several sixes in sequence. Ida explains that the power source is ten miles beneath the planet's surface and they're drilling down to find it. Toby explains that the planet once had an ancient civilization and he copied some of their writing onto the wall from recovered fragments. The Doctor, impressed at their spirit of explanation, gives Zack a hug and then tells them to leave as quickly as they can.
Ida wonders how they arrived and they mention where the TARDIS landed. Zack realizes where they arrived and the Doctor remembers what sections were destroyed. He runs to the TARDIS but finds a sealed door. Looking through the window, he and Rose see nothing but a pit where the TARDIS stood.
The Doctor asks Zack to divert the drilling but he informs them they only have the resources to dig one shaft and they can't make any exceptions. Ida puts them on the duty roster and leaves them alone. The Doctor apologizes for trapping Rose there and they hug.
Jefferson oversees the Ood as they go about their duties. Scooti goes out on the planet's surface to make repairs. Danny makes sure that all the Ood are in their quarters. Toby is in his quarters examining the ancient fragments. As he works, he hears a voice whisper his name. It tells him not to turn around or he'll die. Toby thinks Danny is playing a practical joke and tells him to stop. The lights fluctuate and he goes back into his quarters.
The Doctor examines the ancient writing in the messroom while Rose gets some food. One of the Ood talks to her, saying that the Beast and his armies will rise to war on God. It adjusts its translation device and then says that it hopes she enjoys her meal.
As Zack monitors the drill head, the display screen morphs into the image of a horned beast when he turns away.
As Danny locks the door, the computer voice says "He is awake." When he asks for clarification, it goes back to its standard speech.
Toby returns to his work and the same voice addresses by name. It tells him that if he turns around then he'll die. However, it assures him that it's right behind him and closing in, while reminding him that if he looks he'll die. Toby finally turns around but there's no one there. As he turns back to his work, he senses something on his hands. He removes his gloves and discovers that the alien writing has covered his hands. He picks up a mirror and discovers his entire body is now covered with the writing. He collapses to the floor, unconscious.
In the messroom, Ida pulls back the observation shutters to reveal that the Scarlet System, home of the Pollushi, is being sucked into the black hole. The crew leave and the Doctor and Rose discuss their fate. Rose checks her phone and discovers she can't get a signal because they're out of range. The Doctor admits that he can't build a new TARDIS with his home planet gone. Rose suggests that once they get a lift off of the planet, they try to settle down somewhere and have a normal life. The Doctor is aghast at the possibility and Rose suggests they might share a house. Suddenly Rose's cell phone rings and a voice says "He is awake."
Toby wakes up in his quarters, his eyes glowing red.
The Doctor and Rose go to see Danny, who is overseeing the Ood as they rest. He explains that they're low-level telepaths and the Doctor suggests that they might be picking up low-level telepathic waves. Danny says that there's all kinds of stray transmission. and they monitor the telepathic field. The telepathic field increases and the Ood all turn and look upward. They say that He is awake and they will all worship Him. They refuse to say who is talking through them, however.
Scooti goes to Toby's quarters and hears a message saying an airlock door has opened and closed. She checks with the computer and discovers that no spacesuits have been logged out. The computer starts speaking of the Beast again. Scooti goes to the port and sees Toby outside, without a spacesuit but still living. He grins at her and tries to compel her to come out. When she shakes off his mental influence, Toby gestures and cracks open the port. Scooti tries to run but the port shatters and she's sucked out into the vacuum. The breach causes shocks throughout the base. Everyone manages to get to safety as the computer seals the breach. Toby is among the crew, his skin back to normal. Zack locates Scooti's biochip in Habitation 3 and assumes she's there. Toby stares at his hands, which no longer have anything on them. The Doctor asks him what happened and Toby pleads amnesia. Rose and the Doctor take him back to the messroom. They look through Habitation 3 but can't find Scooti. The Doctor looks up through the shutters and sees Scooti's body above, being pulled into the black hole. The Doctor says he's sorry as Ida closes the shutters.
The drill stops: it's reached 10 miles down at the power source, Point 0. The Doctor volunteers to go with Ida and Zack reluctantly agrees to let him go. Rose tells him to make sure and come back. Meanwhile, Danny confines the Ood to their holding chamber. Zack sends the capsule down with Ida and the Doctor. Rose watches on the monitor as the capsule descends down the shaft past the oxygen field.
In the Ood chamber, the Ood get to their feet and ignore Danny's orders.
The capsule stops and the Doctor and Ida tell the crew that they're okay. They proceed through a massive cavern and finds the ruins of Toby's ancient civilization. Rose passes the news on to Toby, who is nervously studying his hands. Ida and the Doctor proceed toward the power source and discover that their video signal isn't going through.
Danny reports to Zack that the Ood are acting oddly, staring at him. He warns that the Ood's telepathic field is 10 times normal, indicating brain death. Zach tells Jefferson to keep watch on the Ood.
The Doctor and Ida find an enormous seal covered in the writing. They can't find a mechanism to open it and conclude the writing is the key. Zack and Rose ask Toby if he's translated the writing. Toby says he's worked it out and stands up, covered in the writing. He says they're the words of the Beast and He has woken. In the control center, Zack loses radio contact. Meanwhile, the possessed Toby taunts Jefferson, asking if his dead wife ever forgive him. When Jefferson threatens to shoot him, Toby asks how many he can kill. The writing slides off of his body and enters the Ood, leaving Toby back normal. The Ood chant that they are the Legion of the Beast and the ones in the holding bay start to march out. One of them approaches Danny's assistant, speaking of the Beast being known as Satan and Lucifer. It kills Danny's assistant with its translation sphere. Jefferson orders a retreat.
At the bottom of the shaft, the seal starts to open on its own. Zach determines that the planet is moving. He opens the shutters to see that they're falling into the black hole.
The Ood close in on Jefferson and Rose as they try to open the sealed door leading out of the section.
The Beast begins to emerge from the shaft, saying that at long last he is free.