The Tenth Doctor and Donna land on a planet called the Ood-Sphere in 4126, where a company called Ood Operations has been harvesting and selling the Ood as servants. Several people have been killed in the weeks before the Doctor arrived. 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. The Ood involved were also afflicted with red-eye, where their eyes literally change red.
One of the Red-Eye Ood, on display at the Doctor Who Experience.
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. Meanwhile, all the Ood become afflicted with red-eye and begin a mass revolution, fighting back against the guards in the facility. 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. The Ood’s song resonates across the galaxies, and humans decide to return their Ood servants back to the Ood Sphere. 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.