The Doctor (David Tennant) and science officer Ida Scott (Claire Rushbrook) investigate a pit beneath an ancient trap door deep beneath the planet Krop Tor. In the sanctuary base above, the Doctor’s companion Rose (Billie Piper) and three surviving members of the expedition team, Jefferson (Danny Webb), Danny (John Maynard Jefferson) and Toby (Wil Thorp) are fleeing from the alien Ood, who are possessed by the Beast (voice if Gabriel Woolf).

As the Doctor prepares to return to the base, the Beast speaks to the Doctor and the crew through the Ood. The Beast was sealed in the pit before the universe began, though as a Time-Lord the Doctor can’t believe that. He quite effectively demoralizes everyone. Then the cable snaps trapping the Doctor and Ida ten miles underground.

In the base control room, Zach (Shaun Parkes) is cornered by the Ood. The rest are pursued through the maintenance tunnel. Zach seals off the junction to stop the Ood. The rest try to incapacitate the Ood by interrupting their telepathic field. When the group reunites with Zach, he knocks out Rose and takes her with them to the rocket to escape the planet.

The Doctor and Ida use the lift cable to explore the pit but find only darkness. The Doctor plunges into the pit and finds he can breathe at the bottom, in the air-pocket that cushioned his fall. He sees the Beast’s physical form, which is a giant red Satanic demon. It is unintelligible. Which tells him its intelligence has escaped. Krop Tor, orbiting a black hole, was the perfect prison, as it would plunge into the black hole if the Beast escaped.

The Doctor has found two-handled jars that light up, which seem to be ancient batteries that not only hold the demon but hold back the power of the black hole. The Doctor destroys them, and the gravity field collapses. He sacrifices himself, as he is wont to do, to destroy the Beast. Then he finds the TARDIS. The rocket is also pulled toward destruction. Toby is still possessed by the Beast. Rose shoots out the window of the rocket and Toby is sucked out. The Doctor rescues Ida with the TARDIS and tows the rocket to safety. The Ood are lost.

The story was generally well received, though criticized for all the collateral damage. The Beast was impressive, but the plot thought a little simple, with plenty of holes. The story was exciting enough and the Beast thought-provoking enough that most people didn’t notice the inconsistencies until afterwards. The Beast was based on the horrific creatures created by comics artist Simon Bisley. There was a nice mix of good actors playing standard science-fiction characters faced with a moral dilemma and a power beyond space and time—in other words, supernatural.

The Ood cease to be cute when they are invaded by evil and march around with glowing red eyes, electrocuting people. Rose and the crew crawl through dark tunnels pursued by creatures like the doomed crewmen of the Alien movies. Creating this mood was obviously a major reason for the story’s existence. In the end, Rose defeats the great evil by killing a human being, but nothing much is made of this. We are invited to contemplate the nature of evil, but not given much to contemplate. The Beast predicts that Rose will die.