The planet Bellotron is home to a species of nomadic hunter-gatherers who have yet to make contact with any off-world species, and is thus listed as a Grade One protected planet. Unfortunately, it also has an eccentric orbit that, within 36 hours, will take it out of Rutan-controlled space and into a Sontaran-controlled sector. The Earth spaceship Rites of Passage is watching over the planet in the meantime, but when Captain Quilby and Commander Ryan detect an artificial electrical signature from the planet’s surface, they realise that someone has slipped past them. Quilby orders Ryan to send an artificial probe to the planet’s surface to investigate, and the probe finds the bodies of a nomad and his pack animal near a set of caves; the animal has been torn apart and the nomad has been electrocuted, which strongly implies that the Rutans have broken the treaty. But when Quilby sends the probe into the caves, he runs into a problem: the electrical activity they’ve detected is located behind a slab covered in ancient hieroglyphics, and the probe’s software won’t let it risk damage to what could be a significant cultural artefact.
Quilby thus recalls the probe and calls his old friend Irving Braxiatel, asking him to send an archaeologist who can advise them on how to proceed -- and quickly, for the planet will leave their jurisdiction in just over a day’s time. Braxiatel thus sends Benny to Bellotron, but she’s in something of a bad mood when she finally reaches the Rites of Passage, due to her time in cryo-sleep and the complicated security procedures she’s made to pass through when she arrives. She concedes the importance of the security protocols when reminded that the Rites of Passage is currently located between two alien war fleets, one capable of cloning a near-infinite number of reinforcements, and the other consisting of shape-shifters whose touch is lethal.
Benny studies the probe’s video feed, and concludes that the slab is indeed culturally significant -- and also that it can be easily opened up to allow someone through without damaging it. Quilby thus decides to break regulations by sending Benny and Ryan to the planet’s surface, with strict instructions to keep their visit a secret from the locals. Benny and Ryan dress up in thermal suits and descend to Bellotron, accompanied by a squad of C-11 robot soldiers with cloaking capability. Their shuttle glides in silently to avoid drawing attention, but the landing does disturb a pack of brill, fierce carnivorous predators. The brill chase Benny and Ryan to the caves, where they find another pack feeding on the remains of the nomad’s pack animal. Benny demands a blaster with which to protect herself, despite the risk that such an anachronistic weapon might attract attention from the natives. While the C-11s hold off the attacking brill, Benny studies the trip mechanism and works out how to open it -- but in the confusion, she misses a booby trap, and although she and Ryan get through the panel unscathed, half of the robot soldiers are left outside when the panel suddenly closes up again.
As Ryan and Benny proceed through the ancient structure on the other side of the slab, Benny notices that many of the lethal booby-traps seem to have been triggered already. Ryan catches a glimpse of a local nomad in the corridors up ahead, and he and Benny follow the nomad to a tomb with a large sarcophagus. The nomad is apparently trying to steal the burial urn; however, Benny recognises the “nomad” as the same man who’s lying dead back at the cave entrance. The Rutan transforms back into its true shape and attacks them, but the remaining robots destroy it with their built-in flamethrowers. Before Benny can examine the burial urn for herself, a new arrival bursts into the room -- a woman named Bev Tarrant, who’s being pursued by another pack of brill. Bev admits that she’s a thief from offworld who came here to rob the tombs; however, things have become fraught, and in exchange for their protection from the brill, Bev shows Benny and Ryan to an underground river which will bypass the rest of the booby-traps and lead them safely out of the tomb. Benny wants to remain and find out why the Rutan was so interested in the burial urn, but Ryan refuses to do so; as long as the Rutan has been dealt with, his mission is over, and he thus forces Benny and Bev to accompany him back to the shuttle and depart with the robot soldiers.
Back on the Rites of Passage, Ryan locks Bev in his cabin until he and Quilby can think of a cover story to explain her presence -- one that doesn’t involve their breaking regulations and landing on a prohibited planet themselves. Benny points out that Bev’s own ship is probably still on the planet’s surface, and that they still don’t know why the Rutan was so interested in that burial urn. However, Bellotron will cross into Sontaran space within five hours, and there are already ships gathering at the frontier. Quilby thus declares the incident officially over, and sends down an automated probe to find Bev’s ship and pilot it off the planet.
Benny returns to her quarters, upset, only to find Bev waiting there; she’s managed to escape, and she convinces Benny to steal a shuttle and return to the planet’s surface for the mysterious urn. Ryan detects the shuttle’s launch, but when he scans the ship to confirm whether Benny and Bev are aboard, he learns that Bev Tarrant has been locked up in the holding cells pending an investigation by the ship’s security chief. Fearing that their illegal trip to Bellotron’s surface is about to get out, Ryan visits the cells to question Bev, only to find that she doesn’t recognise him and has no idea what he’s talking about...
Down on Bellotron, Benny and Bev return to the burial chamber, but “Bev” then casts off her disguise to reveal that she too is a Rutan. The urn is not a native artefact at all, but a Rutan bomb designed to release a mutagenic virus within Sontaran space. The entire population of Bellotron will be killed by the explosion, but to the Rutan, this is an irrelevant side-effect. However, the bomb has malfunctioned, and this Rutan thus returned to Bellotron to repair it -- unlike the other Rutan, a member of a traitorous faction who had come to Bellotron to disarm the bomb. But as “Bev” prepares to kill Benny to protect her secret, Benny reveals that she also has an important secret...
With only 38 minutes to go before Bellotron enters Sontaran space, Quilby allows Ryan to take one last quick trip back to the planet’s surface to rescue Benny. However, the Sontaran battlecruisers cross the border and strafe the caves just as Ryan arrives, killing both Benny and the Rutan Bev before Ryan’s eyes. Stunned, Ryan returns to the Rites of Passage -- where he’s even more stunned to find Benny waiting for him back on the ship. Just like Bev Tarrant, the real Benny was captured on her way to Bellotron and duplicated by the Rutans -- but while Bev barely escaped from her captors with her life, Benny’s captors were members of the pacifist faction, and once her duplicate had been sent to Bellotron to stop its comrades’ genocidal plan, the real Benny was released unharmed. The Rites of Passage returns to duty, leaving Bellotron to enter Sontaran space, and while Quilby and Ryan ponder the diplomatic and political ramifications of this new pacifist Rutan faction, Benny and Bev retire to the lounge to discuss a common friend who is responsible for Bev being in this particular era of space and time.