The TARDIS materializes in a Sanctuary Base far out in deep space. The Doctor (David Tennant) and Rose (Billie Piper) discover strange symbols on the wall that defy the TARDIS' ability to translate, which indicates that the writing is older than his people might know. They're suddenly startled, however, by a group of bizarre alien creatures - which they later discover are called the Ood - who they believe are attacking them but are, in fact, perfectly docile subordinates (or slaves) to the humans who inhabit this base, including the makeshift captain, Zach (Shaun Parkes), scientist Ida (Claire Rushbrook), archaologist Toby (Will Thorp), security chief Jefferson (Danny Webb), maintenance trainee Scooti (MyAnna Buring) and Danny (Ronny Jhutti) who works with the Ood.

When a massive impact hits the base, the Doctor and Rose realize where they are... an impossible planet, an airless world orbiting a black hole which cannot possibly exist, which in legend is called the Bitter Pill. The expedition is now stuck here, monitoring the black hole in a place where by all scientific probability it - and they - cannot exist. The team is drilling to reach a power source, which keeps the world in balance, ten miles below... something buried down within the planet that they have not yet identified. But soon the Doctor realizes the depth of his misfortune: when the impact hit, it wiped away part of the base including the TARDIS. Now he and Rose are stuck here, forever.

The archaeologist, Toby, has been examining the ancient writing but has also started hearing voices. Other manifestations that something is wrong occur: Rose and Danny both believe they hear the otherwise docile Ood say something they shouldn't, and Zach fails to see the image of an enormous demon in the holograph tank. Toby discovers that the ancient symbols have begun to appear on his hands, and soon he is not in control of his body. Possessed by something terrifyingly evil, he goes outside the base, watched by Scooti, who is then killed when he shatters the window. But by the time the Doctor and company respond, Toby has returned to normal and has no memories of what's happened.

The drill reaches its destination ten miles below, and the Doctor accompanies Ida down the mine shaft, while on the base the Ood are all confined after demonstrating unusual behavior and psychic patterns. Down below, the Doctor and Ida discover the ruins of an ancient civiliation along with a huge engraved coverstone. Back on the base, Toby is once again transformed into a hideous servant of something called the Beast. The Ood are now completely possessed as an army for this 'beast,' which is also called other things in many tongues - including, in the language of Earth, Lucifer... or Satan. Suddenly the planet's gravity field starts to collapse and the world starts moving toward the black hole, while in the depths of the planet, the Doctor and Ida watch helplessly as the coverstone opens and something alien inside starts to become free...