Benny is invited to the planet Tysir IV by Lynton Jellis, a historian in need of a professional second opinion. The planet has been colonised by a mining corporation, but the robot diggers recently unearthed an ancient building beneath the surface, and the colony could end up being illegal if it turns out that the planet was inhabited in the past. Benny makes it very clear that she intends to give a completely unbiased opinion, and Lynton, who seems to be a great fan of hers, assures Benny that he asked for her specifically because he knew she’d resist all pressure to give a politically expedient but untruthful assessment. However, due to security concerns, Lynton will have to accompany her at all times.

Lynton pilots Benny to the site, which has already been excavated to a great depth; it’s larger than Earth’s Grand Canyon, and Benny experiences some vertigo as they descend. She and Lynton don thermal clothing to deal with the sub-arctic external temperature, and set off to examine the find -- a large metal doorway too heavy to be opened by brute force. The diggers were drawn to this site by the buried metal, initially assuming it to be a seam of ore. There are strange markings and controls on either side of the doors, and when Benny examines them, she determines that the controls are musical keyboards and concludes that the markings above them must be musical notation. Once she locates the symbol and key for middle C, she and Lynton are able to play the two keyboards in unison, and the doors open to reveal a vertical shaft heading downwards. Oddly, the controls were just the right size for human hands.

Benny and Lynton descend the shaft to find themselves inside a chamber full of Cyber-technology. The Cybermen supposedly died out long ago, but since their history involves contact with time-travel technology, it’s possible for them to pop up just about anywhere. Benny warns Lynton not to touch anything, but he ignores her and pulls a vial out of a nearby container. As alarms go off throughout the Cyber-complex, Lynton pulls a gun on Benny and reveals that he’s been planning this all along. He was able to crack the code for the base’s external computers and learned that it contained a super-weapon, a deadly plague engineered by the Cybermen to kill human life. He lured Benny here to help him get inside the base, choosing her because of her reputation for attracting trouble. When he leaves the base and blows it up behind him, everyone will assume that the famous adventuress’ luck finally ran out; in the meantime, Lynton will make a fortune selling the plague to the separatists in the Polon system for use as a terrorist weapon.

The alarms have woken a hibernating Cyberman, but Lynton flees before it arrives, leaving Benny with a timer counting down the seconds until the bombs explode. Once awake, the Cyberman reveals that the base was created as a trap to lure humans into the Cybermen’s clutches. It does not understand why Lynton would wish to blow up the base, but when Benny reminds it that humans do not always behave logically, the Cyberman concedes the point and agrees to work with her to escape. Noting that the air in the chamber is fresh, Benny concludes that the Cybermen must have installed ventilation for the benefit of the humans that they sought to lure here, and she and the Cyberman climb up the shafts to the surface. There, they find Lynton lying dead before the hatchway, the first victim of the plague; the virus is not contained within the vial, but coated on its surface, and Lynton was doomed the moment he touched it. The Cybermen intended to lure humans to this base and infect them with the plague; within a year, the virus would have burned itself out, but by that time the human population would have been decimated and the weakened survivors who returned to the base in search of an antidote would have been captured and transformed into Cybermen.