Planet of The Ood

The Doctor uses the TARDIS to land at a random point in time and space. On leaving the TARDIS, he and Donna find an injured Ood, a species the Doctor previously encountered in "The Impossible Planet" and "The Satan Pit". Just before he dies, the Ood's eyes turn red and it makes a lunge for the Doctor, startling him with his ferocity. The Doctor muses that they were being influenced by the Devil on their previous encounter, and concludes that on this occasion they must be being influenced by a different and closer being. The Doctor and Donna find an industrial complex controlled by Ood Operations, who have been selling the Ood as a servant race since 3914. They have even been making certain upgrades to their translation sphere. These include standard voice and a more seductive female voice. One even has been adapted with comical expressions such as "D'oh" from Homer Simpson of The Simpsons. The Doctor locates their position: the Ood-Sphere in 4126 close to the Sense-Sphere of the Sensorites.
The "Red Eye" phenomenon is affecting other Ood on the planet: several people have been killed in the weeks prior to the narrative. During the outbreak, the Ood state that "the circle must be broken". Ood Operations noted an increase in the phenomenon, and considered it to be similar to foot-and-mouth disease; CEO Klineman Halpen tells the Doctor the method of killing is identical.
Throughout the episode, Donna becomes sympathetic to the Ood and is horrified by their status as slaves. The Doctor also takes an interest in the Ood, noting that no species could naturally evolve to serve. He also feels he had overlooked them on their previous encounter. He and Donna travel through the complex and find a batch of uncultivated Ood. Instead of a translation sphere, they hold a "hind brain" that gives them individuality, and once removed, they become subservient; the Doctor castigates Halpen for lobotomising them.
The Doctor and Donna are captured by Ood Operations' security force. Shortly after, the Ood begin a mass revolution, and the complex is evacuated. The Doctor follows Halpen to a locked warehouse. The warehouse contains a large brain, which completes 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 a joint effort between the Doctor, Donna, Dr Ryder, and Halpen's personal Ood, Ood Sigma; Ryder, an activist for "Friends of the Ood", had slowly infiltrated the company over the course of ten years before he was able to gain access to the controls for the pylons and change them to their minimum setting, while Ood Sigma used Halpen's hair-loss medication to slowly convert Halpen into an Ood.
The Doctor shuts down the circle, freeing the Ood and allowing them to all rejoin in a telepathic collective. Before leaving, Ood Sigma promises to include the "Doctor-Donna" in the Ood's song; stating that "the Wind, the Ice and the Snow" shall remember and honour their names forever, but comments that the Doctor's song may soon end.