Doctor: David Tennant (Tenth Doctor)
Companion: Catherine Tate (Donna Noble)
Tim McInnerny – Klineman Halpen
Ayesha Dharker – Solana Mercurio
Adrian Rawlins – Dr Ryder
Roger Griffiths – Commander Kess
Paul Clayton – Mr Bartle
Paul Kasey – Ood Sigma
Tariq Jorden – Rep
Silas Carson – Voice of the Ood
Directed by Graeme Harper
Written by Keith Temple
Script editor Lindsey Alford
Produced by Phil Collinson
Executive producer(s) Russell T Davies
Incidental music composer Murray Gold
Production code 4.2
Series Series 4
Length 45 minutes
Originally broadcast 19 April 2008
The Doctor sets the TARDIS controls to random and materialises on a snowy alien planet. Outside, he and Donna find an injured Ood lying in the snow. Just before he dies, the Ood's eyes turn red and it makes a lunge for the Doctor and startles him with his ferocity. The Doctor surmises that the Ood was being influenced by a being near them. They find a nearby industrial complex called Ood Operations, a company that has been harvesting and selling the Ood as servants. The Doctor discovers that they are on the Ood-Sphere in the year 4126, close to the Sense-Sphere of the Sensorites. The "Red Eye" phenomenon begins affecting other Ood on the planet and several people are killed in the weeks before the Doctor arrived. The possessed Ood keep stating that "the circle must be broken". Ood Operations consider the phenomenon to be a disease similar to foot-and-mouth disease. The CEO of Ood Operations, Klineman Halpen, tells the Doctor the method of killing each time is identical: the victims are electrocuted by the Ood's translation spheres.
Throughout the episode, Donna becomes sympathetic to the Ood and is horrified by their enslavement. The Doctor also takes an interest in the Ood, noting that no species could naturally evolve to be servants. He and Donna travel through the complex and find a batch of uncultivated Ood singing together. Instead of a translation sphere, they hold a "hindbrain" that gives them individuality. This hindbrain is being removed and replaced with the translation sphere by the humans to make them subservient, and the Doctor rebukes Halpen for lobotomising the Ood. The Doctor and Donna are captured by Ood Operations' security force. Shortly after, the Ood begin a mass revolution and the complex is evacuated, but everyone from the complex, including Solana, are slaughtered en masse. The Doctor and Donna escape with some help from the Ood (after convincing them to resist their brainwashing) and follow Halpen to a locked warehouse that contains a large brain, which is revealed to be the Ood's collective consciousness. The brain's control of the Ood is limited by a circle of pylons emitting a forcefield. Halpen plans to kill the brain and by extension all of the Ood, but is stopped by the Doctor, Donna, and Dr. Ryder. Dr. Ryder reveals that he is secretly an activist for "Friends of the Ood", and had slowly infiltrated the company to gain access to the pylons and lower their force field to cause the revolution. Halpen is outraged at his betrayal, so he throws Dr. Ryder into the brain, killing him. Halpen's personal Ood servant, Ood Sigma, has been using Halpen's hair loss medication to slowly convert Halpen into an Ood. Ood Sigma tells the Doctor and Donna that he will take care of Halpen.
The Doctor shuts down the pylons, freeing the Ood and allowing them all to sing in a telepathic collective. As the Doctor and Donna prepare to leave, Ood Sigma promises to include the "Doctor-Donna" in the Ood's song. He also tells the Doctor that his song will soon be ending.