Mr. Bartle, an advertising exec for the Ood Corporation, is going over a new advertisement promoting the use of the Ood race as servant. He's interrupted when one of his own Ood servants uses its translation device, wired into its head, to electrocute him.
The Doctor decides to go to a randomly selected planet. He and Donna arrive on an ice planet, and follow a spaceship coming in for a landing at a nearby facility. The ship belongs to the CEO of the Ood Corporation, Klineman Halpen. He has arrived to inspect the central processing facility of the Ood on their homeworld. Other Ood have gone "Red Eye" and killed their owners. Salesperson Solana Mercurio meets with him before preparing for an executive sales meeting to be held at the facility. Halpen is concerned about his hair loss, and constantly drinks a hair tonic provided by Ood Sigma, his own servant.
As the Doctor and Donna head for the facility, the Doctor hears a telepathic singing in his head that Donna cannot. They find an Ood, dying from a gunshot wound. It talks about "the circle" and then its eyes glow red and it tries to attack them. In its weakened condition it fails, and then dies. The Doctor realizes they're in the 41st century and that he previously encountered the Ood when a creature claiming to be the Devil possessed them.
The duo gets to the facility and the Doctor claims to be a member of the "Noble Corporation." Bluffing his way in with his psychic papers, they join a tour just as alarms go off. Solana claims it’s a work-shift notification and continues on the tour, while Halpen knows better. He orders his soldiers, led by Commander Kess, to catch the renegade Red Eye Ood. Kess finds the missing Ood and discover it has gone rabid. Under Halpen's orders, Kess takes it to Dr. Ryder, the head researcher at the facility.
After the tour, the Doctor realizes they are on the Ood Sphere in the year 4126. Donna talks to an Ood but it says that all of its kind have been born to serve and can do nothing else. The Doctor doesn't believe that, and it starts to speak of the circle but is summoned away before it can finish.
Ryder is examining the rabid Ood and asks for permission to examine the contents of Warehouse 15. Halpen says the contents have been inactive for almost two centuries, but agrees to give him access. Meanwhile, the Doctor and Donna slip away from the sales meeting and slip through the facility. They see the human overlords ruthlessly whipping the Ood and the Doctor resolves to free them from their slavery. They spot Halpen and Ryder and follow them toward Warehouse 15. Inside, the two men check the warehouse's glowing contents and confirm that it's still secure. Solana contacts them with word that the Doctor and Donna have disappeared and there's no Noble Corporation.
As Halpen sounds the alarm, the Doctor and Donna find another warehouse filled with Ood ready for shipment to other planets. They talk with the Ood but the creatures have no interest in leaving and no understanding of freedom. When the Doctor asks them about "the circle," they all chant in unison saying that they can only sing if the circle is broken. They're interrupted when Kess and his men arrive. The two split up and Donna is captured. The soldiers put her in a cargo crate for safekeeping, but as she looks on in horror the Ood's eyes start to glow. Kess then uses an automated cargo claw to play a cat-and-mouse game with the Doctor, eventually pinning him down. Kess brings the Doctor along and releases Donna, who gets out just ahead of the Ood going rabid. The duo makes their escape while Kess and his men open fire on the Ood, and run into Solana. The Doctor manages to convince her to direct them to Halpen, but she betrays them to the guards and they're forced to wonder.
Kess and his men manage to confine the Ood in the warehouse. The commander reports to Halpen, who orders the creatures' destruction.
The Doctor and Donna get to Warehouse 15, and the Doctor seals the door behind them with his sonic screwdriver. They find a cage holding unprocessed Ood, who are telepathically singing. The Doctor opens Donna's mind to them, and she weeps at their Song of Captivity. When the Doctor talks to the Ood, one of them shows him its secondary hindbrain: fastened by an umbilical to its brain. The Doctor realizes that the Ood Corporation removes the creatures' hindbrain to make them docile and non-telepathic, and then replaces it with the translator unit. The creatures are normally peaceful, since their hindbrains are external. Donna is disgusted at this glimpse of a harsh future, and says she wishes to be taken home. However, Halpen arrives and has his men take them into custody. In his office, he defends the corporation's practice of slavery and tells Kess to wipe out the entire lot with a canister of poisonous gas.
The unprocessed Ood telepathically contact their enslaved brethren, who all go Red Eye. The Ood attack the security teams and the executives, killing many of them along with Solana. Several of them get to the processing warehouse and lock Kess in a cell without his mask before leaving. The gas is released and Kess is killed.
Halpen has Donna and the Doctor handcuffed to the wall and prepares to leave. The Doctor notes that the Ood must have a third brain and Halpen admits he's right. However, he doesn't care and leaves with Ryder and Ood Sigma. Outside, Halpen considers the devastation and contemplates Sigma's loyalty, then releases it to its people before continuing on to Warehouse 15 to destroy its special contents.
More Ood arrive in Halpen's office and advance upon the Doctor and Donna. They shout out that they're "Doctor-Donna Friend" and the unprocessed Ood sense them and halt their processed brethren just in time. The Ood release the duo, who head for Warehouse 15 but run into Sigma.
Halpen and Ryder enter Warehouse 15 and the CEO orders the scientist to destroy the contents. The Doctor, Donna, and Sigma arrive and see what the warehouse's contents are: a giant Ood brain, contained within a force field. The Doctor realizes it's the third and most important part of the Ood collective consciousness. Halpen explains that his family found the Ood 200 years ago and captured the third shared brain, then cut it off from the Ood consciousness to enslave the creatures. He doesn't understand why the Ood are going rabid after all this time, but he prepares to destroy the third brain.
Ryder interrupts, revealing that he's a member of the Friends of the Ood, a protest group, and he's spent years infiltrating the corporation. He's been lowering the force field gradually, giving the Ood the chance to fight back. Halpen throws him over the railing into the brain, killing him, then prepares to shoot the Doctor. However, he hesitates and Sigma offers the CEO his hair tonic. Too late, Halpen realizes that Sigma was free-willed all along, and has been slipping Ood biological content into his tonic. The last of his hair falls out and then he transforms into an Ood, a new hindbrain slipping out of his head and into his hands.
The Doctor shuts down the force field and the Ood consciousness is finally complete, on Ood-Sphere and across the galaxy. The other Ood will return home by rocket and the Doctor prepares to leave. Cheered by the outcome, Donna decides to stay with the Doctor. However, as they leave, Ood Sigma warns that the Doctor's song may soon end. However, he promises that all of the Ood will sing of Doctor-Donna Friend forever.