As the TARDIS materializes somewhere on Earth, Adric argues with the Doctor about the lack of respect he believes he receives compared to Nyssa and Tegan. They explore some mysterious caves and are caught by soldiers under the command of Lieutenant Scott (James Warwick). Professor Kyle (Clare Clifford) accuses them of killing the rest of her archaeological team. The Doctor convinces them otherwise and Kyle leads them to the bodies of her team. Nearby is a metal hatch. Androids attack them, killing some of Scott’s soldiers, but the Doctor defeats them. He thinks the androids are guarding the hatch and manages to open it, revealing a planet-buster bomb. They defuse it and trace the signal to a freighter just entering the solar system. Scott and Kyle accompany them in the TARDIS and materialize on the freighter. The Doctor and Adric explore the ship and find corpses before they are caught by Security and taken to Captain Briggs (Beryl Reid).
All this time, they are watched by the Cybermen. With the bomb defused and their plan interfered with on the ship, the Cyberleader decides it’s time to take over the freighter. His troops had been stored in sealed containers, which they now leave and march to the Bridge. The ship’s crew, with Nyssa, Tegan, Scott and Kyle, try to stop them, but are overpowered, Kyle killed, and Tegan captured. The Cybermen take control of the Bridge. Using Tegan to control the Doctor, the Cybermen lock the controls of the freighter after setting it on a collision course with Earth, expecting the antimatter engines to destroy the planet. The Cybermen order the Doctor to take them to the TARDIS to escape. Adric, knowing the Cybermen are allergic to gold, passes his mathematics star to the Doctor.
Scott and his men overpower the minimal guard on the Bridge and Adric begins to undo the control lock. His first attempt causes the ship to jump back in time 65 million years. His second attempt brings the ship out of warp, though still on a collision course with Earth. Briggs, Scott, and the crew leave in escape pods. They convince Adric to leave with them, but he comes back to try one more thing to undo the control lock. The Cyberleader orders the TARDIS crew killed, but the Doctor smashes Adric’s badge into the Cyberleader’s chest plate and kills him, allowing the crew to disable the Cybermen.
The Doctor finds the TARDIS controls damaged, so he cannot rescue Adric. On the Bridge, one last dying Cyberman fires at him, destroying the keyboard. The TARDIS crew watches in horror as Adric is killed in the explosion that wipes out the dinosaurs.
The story was quite atmospheric and creepy, with low lighting and impressive Cybermen. A series of flashbacks includes all the previous encounters of the Doctors with the Cybermen, in black and white. The Nostromo set for Alien provided parts for the modelmakers. The story was highly praised and is a favorite of Peter Davison. It was a great farewell for Matthew Waterhouse, who had not been given great things to do before and was not exactly a fan favorite. As someone said, “Why do they think fans like child geniuses? We hate them. Star Trek found that out.”
No long-running companions had ever died. Katarina (Adrienne Hill) was a companion of the First Doctor. She was an inhabitant of ancient Troy and would have been so difficult to write for, as she had no scientific knowledge whatsoever, that she was tossed out of an airlock by the Daleks. Sara Kingdom (Jean Marsh), a security officer in the 40th Century, born on Mars, who travelled in the TARDIS for one story, got caught in a destructor field and aged to dust. I’m sure she had better things to do in her acting career, though she did return as a villain with the Seventh Doctor. She is not formally listed as a companion. So, the death of Adric, who had been a companion in eleven stories with two Doctors, was a big deal.
The story ended with silent credits over a shot of Adric’s shattered badge. The producer, John Nathan-Turner, was so determined to keep all this secret that he made no mention of the Cybermen in the newspapers, closed the viewers’ gallery at the studio, and even listed Adric in the credits of the following week’s episode.