Jo is shopping one day when she sees a spaceship land — right in the middle of London. Two blobby aliens approach her and seem to know who she is. As she tries to communicate, she has trouble saying certain words.
Sergeant Osgood is left in charge of UNIT HQ, and notes an alien invasion. As he tries to inform the Doctor, the two of them also have trouble communicating. The Doctor notices that they can only say words using the vowel "o", and, as Osgood notes, sometimes "y".
While the Doctor and Osgood try to solve the mystery of the missing vowels, the aliens, known as Forty and Forty-Two, kidnap Jo and take her to a dark room where Forty, Forty-Two, and their leader, Two, subject Jo to a history of their people, the Kobolds. Jo realises they blame the TARDIS for the religious wars on their planet.
The Doctor and Osgood head for the Doctor's hut to work on a device that will block the Kobolds' signals. However, the Doctor becomes frustrated with Osgood and storms into the TARDIS, where he learns he can speak normally.
The Kobolds take Jo to the Doctor's hut, and Osgood tries to warn the Doctor. When Osgood asks, "Why?", the Doctor understands what has happened to the vowels. The Kobolds break in and attack the Doctor, and after they seemingly overpower him, they head inside the TARDIS. However, this is part of the Doctor's plan.
Once inside the TARDIS, the Kobolds begin speaking normally. But Osgood enters with the Doctor's proton box, which explodes, killing Two and Forty. Forty-two tries to take over the TARDIS, and the Doctor needs Jo to distract him. At first nothing works, but then Forty notices the roundels, and is distracted long enough for the Doctor to take him back to his planet, Procyon Two.
The Doctor later explains to Jo and Osgood that the TARDIS had once materialised on Procyon Two, and had begun the process of changing the native language to English, but as the Doctor was still exiled on Earth, the Time Lords yanked him back to Earth before the process could be finished. Therefore, the Kobolds could only use the vowels "o" and "y".
- The Brigadier is on leave.
- The Doctor thinks the Zygons or Ssorg might be responsible.
- The Kobolds have met Ky of Solos and Lytton.
- A footnote attached to a moment where the Doctor notes that the TARDIS "took a bit of a tumble during our recent battle against the Master." clarifies that the remark refers to The Time Monster, thus this story takes place after that.
to be added