Benny’s friend Caitlin Peters recently moved to the planet Karthos with her husband Michael, and she’s written to Benny claiming to have found evidence of an ancient civilisation on that world. However, she hasn’t been in touch since, and Benny, worried, decides to take Braxiatel’s shuttlecraft to Karthos to see for herself. As she approaches the Peters’ habipod, she sees several ships deliberately flying towards a huge fireball in the atmosphere, but as she now has a baby on the way, she can’t risk investigating personally. She touches down near the habipod, but nodoby answers her knocking at the door -- and as she shivers in the cold, wondering what to do next, a swarm of giant, shrieking bat-like humanoids descend from the sky and attack her.
Caitlin’s former boss, an environmental scientist named Konstantin, is currently conducting an autopsy on one of the bat-like “fireflies” even as its fellows hammer at the outside of his own pod. They’re nasty creatures, over seven feet tall and massing a hundred kilogrammes, with sharp, serrated talons on their hands and feet. Konstantin is eventually forced to call the crisis centre, and Michael Peters responds and drives off the attackers; however, he’s upset when Konstantin casually dismisses him without thanks. In fact, Konstantin has been so busy that he barely even seems aware that Caitlin has been missing for three weeks. They depart on poor terms; the fastidious Konstantin is impatient to return to work and irritated with Michael for tramping dirt all over his floor, while Michael is angered by Konstantin’s lack of concern for Caitlin. Konstantin gets back to work, and soon detects thulium in the firefly’s blood; could it be that the element is vital to their survival, and that the mining operations are somehow disturbing their life-cycle? As he considers this possibility, the fireflies arrive at his pod again, but this time they do not attack -- and Konstantin collapses in agony, hearing thousands of voices whispering in his mind.
Benny can’t run far in her condition, and she seems to be easy prey until the fireflies break off their attack and withdraw for no apparent reason. Benny doesn’t question her good luck, as she has no way of knowing that a voice in the fireflies’ minds was speaking her name. Michael then arrives, but he and Benny aren’t very fond each other and Michael only reluctantly lets her into the habipod when she refuses to leave without speaking to Caitlin. When she realises that Caitlin is gone she immediately hopes that she’s finally seen sense and left Michael, and Michael angrily reveals that Caitlin never returned from her expedition to the mountains. He too is starting to fear that Caitlin has left him, for things haven’t been going well between them recently. They came to Karthos so Michael could make his fortune mining thulium from the firestorms, but he didn’t realise just how many other miners had come here with the same idea in mind, and he’s had to work day and night just to make enough money for food. After Caitlin took the job with Konstantin, they hardly saw each other any more. Benny realises that Michael is genuinely upset, and finally admits that when she first advised Caitlin not to marry him, Caitlin gave her a black eye. Caitlin would never have left Michael, no matter how bad things got between them -- which leaves only the more worrying possibilities open.
Benny decides to begin her investigation with the ruins Caitlin found. As she checks out the encrypted data on Caitlin’s laptop, Michael tells her about the fireflies, which only began to show up after Caitlin had gone. They’ve been killing about half a dozen colonists per day, and the miners must divide their time between storm chasing and volunteering for rescue duties at the crisis centre. For some reason the fireflies seem to leave Michael alone, and thus he’s been far too busy to go up into the mountains and look for Caitlin himself. He agrees to take Benny to see Konstantin the next morning, but he doesn’t hold out much hope, particularly as Benny, to his mind, seems more interested in the holophotos of cave paintings which Caitlin had stored in her laptop.
Konstantin doesn’t answer when they arrive at his pod, and when Michael uses the crisis centre override codes to enter, he and Benny find Konstantin out cold on the floor. He wakes, dazed, murmuring both Benny’s name and something about authority, but when he fully recovers he has no idea who Benny is. He soon regains his condescending attitude and tries to get back to his work, but Benny convinces him to give her the satellite scans which first attracted Caitlin’s attention. Benny sees a bridge over a canyon and realises that it’s no natural formation, but Konstantin refuses to accompany her or Michael; however, he does give her a Geiger counter, as it may be possible to detect the fireflies by the thulium in their bloodstream. Before going, Benny asks Konstantin’s opinion on the cave painting stored in Caitlin’s computer; the analysis of the minerals in the rock confirm that the photo was taken on Karthos, and it depicts two humanoid figures surrounded by “fireflies”. One of the figures is wearing a crown and has lines of energy coming out of its head, while the other has energy going in. Something about the painting speaks deeply to Konstantin, and when Benny theorises that the set-up is similar to the Ostaran Council -- a government in which a Council leader reports the will of the people to a figure of Authority -- Konstantin suddenly decides to accompany her and Michael to the mountains after all.
Benny is bewildered by his sudden change of heart, but Michael accepts his help, impatient for action and upset with Benny for paying more attention to Caitlin’s studies than looking out for Caitlin herself. They set off for the mountains in Michael’s flyer, but as they approach the canyon with the bridge, hundreds of fireflies suddenly swarm out of the canyon, heading for a thulium storm in the upper atmosphere. Konstantin’s theory that the fireflies are feeding on the thulium is disproven when the fireball grows larger; the fireflies are feeding the storm, rather than the other way around. The fireflies then turn on the flyer, and Konstantin begins to insist that they land. When Michael refuses, Konstantin attacks him; fortunately, their autopilot kicks in at the last moment, and although the flyer crashes it remains more or less intact. Benny and Michael recover to find that Konstantin and the fireflies have gone; now, all they can do is activate the flyer’s distress beacon and set off, on foot, to rescue two people rather than just one...
Elsewhere, the voice of Authority -- the voice of Caitlin Peters -- greets Council, Konstantin, upon his arrival. Council must birth more of themselves; they must be more if People are to die. Council assures Authority that all is well; their first is above, their second is in the forge, their third is in the rock. Soon, their second is birthed, and their third goes to the forge; their fourth, fifth, sixth and more are in the rock. They are legion, and People must, and will, die. But Authority’s host seems insistent upon something, and Authority, who has no idea that “Cat” is short for the name “Caitlin”, can only conclude that an animal like a cat has a message to deliver to a People. She cannot resist the urge to deliver this message, and as Council’s first and second have gone now, he must birth an Only mind to find a People and bring it to Authority to hear the Cat-like’s message. Once the message is safely delivered, then the People can die.
Benny and Mike have enough equipment for their journey, explosives included, but Benny has no intention of going all the way to the mountains now. Caitlin went missing shortly before the fireflies appeared, and they’ve just seen hundreds of fireflies swarm out of the canyon; thus, Benny intends to begin her search there. Beneath the bridge they see a cave entrance, and Mike has to stop Benny from trying to abseil down to it; despite her determination to save Caitlin, her pregnancy is too far advanced to risk strenuous physical activity, and Mike will have to lower her down on a harness. She reluctantly accepts his help, but before they can descend a second wave of fireflies swarms out of the canyon -- presumably the “second” mentioned earlier by Council. As before, the fireflies leave Mike and Benny alone, and soon the way forward is clear again. Mike lowers Benny to the cave and then abseils down after her. As they proceed, the Geiger counter registers large amounts of thulium in the rock, and when Benny shines the torch on the walls, she and Mike see a firefly apparently buried inside the rock. It begins to move as they watch, and pulls itself free; but before they can flee, it speaks, identifying itself as an Only, and telling them to follow it to Council and Authority.
Benny and Mike follow the Only firefly to a larger cavern, where, it falls apart, its mission complete. It’s led them to Caitlin and Konstantin, and Caitlin is wearing a crown which Benny recognises from the cave painting. As Authority, she demands to know whether Benny and Mike are People, and when they confirm that this is so, Caitlin is permitted to deliver her message to them. Caitlin is in great distress, but Council has been ordered to kill the People if they try to interfere; thus, Mike must stand by helplessly as Caitlin reveals that she’s trapped by the crown, unable to escape. The fireflies are a weapon manufactured by the original people of Karthos, disposable soldiers built from the rock and sent out to fight and die as needed. When civilisation on Karthos died out, only the weapon remained, waiting for the command to build new fireflies out of raw carbon and thulium. Caitlin tried to use the fireflies to frighten off Michael’s rivals and generate more thulium storms for him to mine, but the weapon doesn’t understand what she’s trying to tell it, and the fireflies have been killing people.
Authority takes command once again, and as she and Council discuss their plans, Benny theorises that Caitlin has become the Authority which controls the fireflies while Konstantin, who has been studying the fireflies and knows more about them than any other human, has been selected as their representative, or Council. Both have been subsumed by the crown’s power, and the fireflies thus intend to destroy all People they can find. Benny and Mike make a break for it, and Authority lets them run; there’s nowhere they can escape to. Benny is starting to reach this conclusion herself, for if the fireflies can create themselves out of raw carbon and thulium, then any passing spaceship which uses thulium for fuel could provide the fireflies with a step on towards the nearest inhabited planet, and the next; and almost every ship in the galaxy uses thulium as fuel. The fireflies’ creators may have been more discriminating, but Caitlin isn’t in full control of the weapon, and the fireflies could very well attempt to kill every humanoid being in the galaxy unless Michael and Benny can stop them here and now.
Benny returns to the main cavern to speak to Authority and Council one more time. Before they can kill her, she asks them to look at each other and see what they really are, and as she’d hoped, the confusion kicks in. Authority and Council realise that they are People, and People must die; Council must kill People, and Council is a People, so Council must kill Council... The weapon’s confusion weakens its hold long enough for Caitlin to take the crown off at last and throw it away. Freed, she and Konstantin flee back to the cave entrance with Benny; Mike has finished planting his explosives in the cavern, and after seeing Konstantin and Caitlin up the rope to the bridge, Michael hurries up himself and then lifts Benny to safety. Confused and uncertain without Authority to direct them, the fireflies are unable to stop the explosives from going off and burying the cave.
With their controls lost, the fireflies try to focus their willpower through Konstantin, but he manages to resist their control and save the others. A flyer arrives in response to Michael’s distress call, ferrying them all back home at last. Michael has now seen just how far Caitlin is willing to go for love of him, and even though Caitlin is upset by what she inadvertently released Benny assures that her intentions were good. As Benny returns to the Collection, her baby starts to kick, with enough force to confirm that it takes after its father. All seems resolved, but elsewhere, Konstantin’s voice echoes through the remains of the fireflies’ group mind. They know that the crown is the key, and the weapon hasn’t been entirely destroyed yet.