Benny and Jason are vacationing on the planet Phaaag Zenbrou, where the visiting Isaac Summerfield is looking after Peter while Jason and Benny try to conceive a child of their own. Following the destruction of all life on the Mimsphere, however, Benny is torn between her desire to have a child with Jason and her fear of bringing new life into a world where nothing is safe. As she ponders this, she is contacted by a Mim diplomat named Lwpha who asks her to help him rescue his children from the planet Proxima Longissima. During the war, eight million Mim evacuated from this colony world, and in the confusion and panic, a colony of Mim children -- a borogove -- was accidentally left behind. Benny advises Lwpha to appoint her the children's godmother, which gives her a legal pretext to inquire about them. Jarith Kothar, the Draconian currently in charge of the Braxiatel Collection, denies the presence of a borogove on Proxima Longissima and advises Benny against visiting the planet, claiming that the presence of surviving Mim insurgents would make it too dangerous. Nevertheless, Lwpha convinces her to investigate his claim in person, to find evidence of the borogove's existence, and to expose it to the galaxy so the Draconians will be forced to let the children go.

Benny and Lwpha travel to the Mimsphere inside an old space wreck; the planet's orbital defences classify it as junk and allow it to burn up in the atmosphere, and the survival pod hidden inside splashes down safely in the ocean and is hidden amidst the debris. Lwpha takes the form of a Draconian soldier, and he and Benny head for a research pontoon set up above the borogove's last known location. On the way, Benny inquires about Mim biology, and although Lwpha is reluctant to reveal too much to her, he does explain that the Mim, like all forms of sentient life, evolved from a collection of cells that learned to co-operate with one another. Most life in the galaxy evolved from sponges; on the Mimsphere, sponges themselves evolved the ability to shape-change as a defence against predators. Mim reproduce asexually by budding, and their young group together in a cluster where they slowly develop into individual sentient beings, trained to mimic any form of life, survive in any environment, and even reform themselves if cut into pieces.

Lwpha delivers Benny to the pontoon, claiming that she's a biologist named Frymer who was stranded on the planet when the Draconians invaded. The Draconians test Benny's blood, but accept Lwpha at face value, since it's too time-consuming to test every Draconian soldier on the planet and it's assumed that Mim civilians would be unable to mimic a Draconian convincingly. Benny is put to work with Lady-Captain-Savant Cheset, one of the few Draconian females permitted to pursue an intellectual career; she's studying Mim biology in order to figure out how a mass of undifferentiated tissue can think, and she believes that the borogove has migrated from its original location and has since been hidden. Benny is horrified to learn that Cheset has been conducting an autopsy on a dead Mim child that she claims to have found on the seabed, but Lwpha believes that it probably died accidentally when the borogove migrated. The official Draconian stance is that the Mim only mimic sentient behaviour, and that they abandoned their borogove because they don't care for their children; this is presumably to make the destruction of the Mimsphere more palatable to the rest of the galaxy. However, Cheset is interested in finding out the truth, not just supporting official policy. Benny wonders whether Cheset would make a useful ally, but before she can decide, the pontoon explodes.

Lwpha's body shields Benny from the brunt of the explosion, but he is widely scattered, and Benny is left treading water alone for hours until he pulls himself back together. When they are seen together, they are contacted by a cell of insurgents who had no idea that there were other Mim agents aboard the pontoon when they blew it up. The saboteur, Spong, smuggled explosives onto the pontoon's supports by dividing his body into nine pieces too small to trigger the Draconians' perimeter defences. He and Benny try sharing jokes, but find that their species' sense of humour doesn't translate well. Once Benny has recovered from her exposure to the elements, Lwpha takes her to see the borogove -- and, astonished by its size, she realises that there's something Lwpha hasn't been telling her. He is forced to admit that, out of the thousands of offspring in the borogove, only a few hundred will survive to adulthood. This is a natural part of Mim biology and childcare; caring for each individual bud in the earliest stages of growth would be like caring for each individual gamete in a human body, and some human embryos. The Mim care for every child that develops to term, and even though many Mim contributed offspring to this borogove, Lwpha loves each of them as if they were his own. The Draconian position that the Mim don't care for their children simply isn't true -- and the fact that this borogove was abandoned implies that the mass panic during the evacuation was somehow manufactured.

Lwpha and Spong escort Benny to the human embassy, and on the way, she tells them about her future child Keith and the sense of loss she experienced when that future seemed to disappear before her eyes. Lwpha admits that he'd probably prioritise saving other Mim's children, because he could, genetically at least, replace his own. Their boat is then spotted and blasted out of the water, and although Benny and Lwpha manage to abandon it in time, Spong is severely burned and loses most of his soft tissue. As he dies, he admits to Benny that his people have taken deliberately silly-sounding names in order to sound harmless to humans, and Benny tries to offer him comfort in return by telling him about human beliefs in the afterlife. She and Lwpha are now stranded without a boat in the middle of the ocean; Mim insurgents won't risk trying to rescue them now that they've been spotted, and Draconians won't be searching for survivors. Benny thus broadcasts a photograph of herself from her camera-phone, relying on chance to determine who finds them first; fortunately, a boat from the human embassy arrives just seconds ahead of the Draconians, and although the Draconian gunboat harasses them all the way back to the embassy, they're unwilling to risk a diplomatic incident by firing first.

Once safe in the embassy, Benny releases her photographs of the borogove to the galactic media. But the Draconians unexpectedly respond by revealing that there are several other abandoned borogoves on the planet. Records of their existence and the lives of their parents were presumably lost in the destruction of the Mimsphere, and now the public perception will be that the Draconians have a moral right to raise the borogoves, which the Mim obviously abandoned to save their own lives. The Draconians offer to send Benny back home with no repercussions, but Benny risks her life by allowing Lwpha to shed most of his mass and diffuse into her bloodstream so she can smuggle him off-world. She is shaken when her blood is tested at the spaceport, but somehow she passes the screening and is able to get back to the Collection, where Lwpha diffuses back out into her bathwater. Once he's reformed, Benny tells him that the Collection will be hosting negotiations between the Draconians and Mim on the subject of the borogoves, and Lwpha thanks her for her bravery, which may have saved thousands or even millions of Mim children.

Benny's period is a week overdue, which may mean that she and Jason are going to have a child. But despite her efforts to put Lwpha's murder behind her and concentrate on her own family, she can't bring herself to abandon a million Mim children to their fate. She thus arranges an audience with the Draconian Logistician Korenthai, drops a few unsubtle hints that she's inquiring after the Mim's welfare, and is soon invited to have an informal chat with Kothar. With Draconians, it's all about the subtext, and what appears on the surface to be a conversation about fencing and history is always about the welfare of the children. Kothar surprises Benny by revealing that the Mim have already turned down an offer of compromise; they want complete control over their children's future, and will take nothing less. Benny realises that Kothar wants to win her over to his side, but she points out that Lwpha may have been murdered to stop him from probing the evacuation of Proxima Longissima. He already suspected that the panic that overtook the evacuees was unnatural; it could have been caused by the use of phase cannons on a low-level setting, and Kothar had publicly advocated the use of phase cannons during the war. That was just supposed to be a bluff, but it's possible that they were actually put into use -- and someone capable of using weapons of mass destruction on a civilian population, even on their lowest setting, may also be capable of authorising the use of whatever weapon destroyed all life on the Mimsphere.

Benny can tell from Kothar's reaction that this genuinely hadn't occurred to him, and that he's only now realising what a terrible position he'll be in if it turns out to be true. She thus agrees to work with him to find out who is responsible. The only other people with access to Kothar's command codes are Logistician Korenthai, Baron-General Lashenkavaar -- and Kothar's wife Ithva, his "loyal left hand." Surprised, Benny arranges to speak with Ithva, and once again must do a subtle dance of body language to figure out whether Ithva is capable of committing a war crime. Benny thinks their superficially polite conversation is going very well until Ithva tires of it and snaps Benny up against the wall with a garrotte to her throat, as a friendly warning to back off. Word has come down from the Empress's Hand that not a single child is to be abandoned. To her astonishment, Benny learns that just as every male Draconian has his place in a rigid hierarchy based on honour, so does every female have her place in a hierarchy based on stealth. The left hand is the hand that fights dishonourably; Ithva is a spy and assassin, as is every high-ranking Draconian female; they do what is necessary when honour isn't enough, and the men all pretend not to know.

Benny still refuses to give up, especially now that she realises Ithva can help make her case to the Draconians -- if Benny can persuade her to. Since Ithva also wishes Benny to make the Draconians' case to the Mim, she invites Benny to take Peter along to the nikhol vakarshta, a ceremonial retreat for Draconian mothers and daughters, which Ithva is due to attend with her youngest daughter, Sharintha. Although Benny suspects that the ceremony could be dangerous for Peter, she knows that she has to prove to the Draconians that she loves every child equally and not just her own, just as Lwpha cared for all the children in the borogove. She and Jason thus travel to the planet Montavadros with Peter, and Jason remains in the city, watching Benny's back as best he can while Benny and Peter accompany Ithva and Sharintha to the retreat. Before going, Benny takes a pregnancy test, which is inconclusive. Jason also proposes to her, but she turns him down, on the understanding that this is not a rejection but a postponement.

The camp in the Tembleth Desert is more of a caravan park than a ritual retreat, and the ceremony has also been watered down; the daughters will still be taught how to fight for the first time, but the ritual abduction attempts are staged by actors who run away at the first sign of trouble. Ithva reveals that she and a few like-minded women have arranged a more traditional retreat; they will camp out at a mesa in the desert, and will be attacked by hired thugs who will genuinely attempt to abduct the children. Benny is unwilling to put Peter through that, but Ithva reminds her that she's doing this for all the children, not just her own -- and when Peter complains that the camp is boring, Benny reluctantly agrees to Ithva's terms. Four other mothers accompany them out into the desert; Marthaleka, Rathklin, Salthmanika, and Misilvar, all of whom hold high positions in the secret hierarchy of the Empress's Hand. On their way to the mesa, Benny learns that the nikhol vakarshta dates back to the visitation of Karshtakavarr, "The Oncoming Storm," a traveller in time and space who saved the Draconians from a great space plague. He attempted to take the Empress's daughter with him as he left, but the Empress herself drove him off empty-handed in a fight that has come to represent female empowerment and the importance of protecting the children, even from those who believe they have honourable intentions.

Benny does her best to fit in with the other women, and helps to slaughter a droth, one of the pack animals who carried them out to the desert; they need the meat for food, and the flammable blood for the camp's firebowl. However, when she glimpses Ithva talking with one of the other mothers, she realises that Ithva's natural body language has nothing in common with human expressions, and becomes concerned that Ithva has been putting on an act even when Benny thought her guard was down. Benny thus breaks the rules and warns Peter in advance to expect trouble, but he accepts her claim that she had to put him in danger to save a lot of other children's lives. That night, the bandits attack the camp, and the Draconian mothers automatically group together, already knowing what to do, while Benny and Peter flounder in the dark. But Benny has prepared for this by burying strips of sheeting soaked in droth blood, and when the men move in, she sets the sheeting alight, creating a ring of fire around the camp and destroying the bandits' element of surprise. The women all work together to drive off the bandits, and seem to accept Benny more fully into their circle after the battle. Sadly, when Benny wakes the next morning, she finds that she's miscarried. She tells Ithva what's happened, and Ithva, genuinely horrified, realises that Benny believed so strongly in what she was doing that she was willing to risk her own possible unborn child for the sake of the Mim children. Ithva admits that she too has lost a child in the past, and when she shows Benny the Draconian expression of grief, it simply doesn't translate into human terms. Ithva hasn't been faking her body language to keep her true self hidden, she's simply been translating it into human terms just as she's been speaking -- and even thinking -- in English. Benny now knows that Kothar deliberately set her onto people whom he officially regarded as above suspicion, but she's starting to hope that Ithva isn't responsible.

Salthmanika lays out a Draconian tarot and identifies Benny as the waterflame, the reflection of dawn sunlight on the ocean -- fire in water, a contradiction that symbolises the changeable nature of reality. The Draconians know that their system of honour is a sham based on the whims of the most powerful, but they still need it to make sense of a chaotic universe. When Rathklin comments that their society allows the best to rise by winnowing out the weak, Benny compares this to the nature of the borogove. Rathklin is put out by this small victory of Benny's, and when the men attack again while Benny and Rathklin are on watch, Rathklin points out that Peter is in danger. Benny immediately rushes to his rescue, but Marthaleka was already moving to save him, and by abandoning her lookout post Benny nearly allows two bandits to circle around behind the women. Fortunately the bandits are all driven off, but Benny has damaged her cause by running to save her own son rather than working with the rest of the team to protect all the children at once. Ithva tells Benny that Karshtakavarr's visit showed the Draconians that there were more advanced species out in the galaxy who would take away the Draconians' children with what they believed were good motives. The greatest blow to Draconian society in recent times came when they had to ally themselves with the humans against a far more ruthless species -- because that meant putting the fate of their children in alien hands. However, Benny points out that this is just what the Draconians expect the surviving Mim to do, and Ithva concedes her point.

On the last evening of the retreat, the women hold a dedication ceremony around the firebowl, stating their identities and their mission statements as mothers. They sing, they stand watch, and finally Benny goes to sleep with Peter by her side... and wakes the next morning to find that everyone else in the camp, mothers and daughters alike, has been slaughtered. Ithva, standing watch on the mesa, saw no sign of the massacre, and she's the only Draconian who's been spared. An old human male then appears at the edge of the camp, holding Sharintha hostage, and reveals that Benny clung onto Peter so tightly as she slept that he couldn't separate them. He's killed the droth and bribed the bandits as well, leaving Ithva and Benny stuck out in the desert with no way back and no way to save their children -- unless they agree to stop their investigations into the use of phase cannons on Proxima Longissima, and allow the Draconians to take control of the borogoves' destiny. Benny thus learns that Ithva has also been investigating on her own since one of Kothar's clerks, Pashar, disappeared back on the Collection. The Mim were indeed induced to abandon their borogoves in a state of panic, but the Draconians weren't responsible. The kidnapper produces Salthmanika's tarot deck and invites Ithva to draw a card -- and when she does so, she sees the face of the kidnapper on the card identifying Karshtakavarr.

The kidnapper threatens to kill Sharintha and Peter before their mothers' eyes unless they agree to his terms, but Benny calls him out; she's met the real Karshtakavarr, and knows that while he sometimes commits necessary evils, he's never cruel about it. As the kidnapper hesitates, thrown by her certainty, Ithva attacks him and frees Sharintha from his clutches -- and he changes shape while under attack, revealing himself to be a Mim. Caught out, the Mim attempts to kill Peter and Sharintha, but Ithva and Benny find the strength to run away from their children -- towards the firebowl, which they dump over the Mim's body. The fire burns away his soft tissue, and Benny and Ithva keep him down until he's dead. The bandits then arrive to finish the job, but Benny and Ithva manage to fight them off until the cavalry arrives -- Benny's loyal left hand, Jason Kane, in the back of a hover-taxi from the spaceport city.

The Braxiatel Collection is hosting the reparation talks between the Draconians and the Mim, at which the future of the borogoves will be decided. The Mim diplomatic party is led by Dr Mwshi, and includes Phwmi, leader of a pacifistic faction that intends to bring up the next generation of Mim to abjure all forms of violence, even if it means teaching them that the previous generation deserved to die for their warmongering ways; it also includes Sherm, a wealthy philanthropist who hasn't explained exactly what he was doing off-world when disaster struck the Mimsphere. The Mim have already rejected all Draconian offers of compromise, as Mwshi seems to be under the thrall of Mr Szmyt and Ms Haddad, mysterious "advisors" from a human Institute outside the government and beyond the police. Haddad goes so far as to make a veiled threat against Peter if Benny keeps interfering. Nevertheless, Benny tricks Mwshi and Kothar into meeting informally at the pub by inviting them both separately. Mwshi doesn't want to deal with a species he believes is responsible for committing genocide, while the destruction of the Mimsphere has put Kothar in the awkward position of fighting to keep control of the borogoves in order to justify the Draconians' war against the Mim. Benny suggests a compromise in which the Mim take care of the children with Draconian funding and support, and the Draconian Emperor cedes Proxima Longissima back to the Mim once the borogoves come of age. Kothar and Mwshi agree to this in principle, but then Ms Haddad shows up at the pub, and Mwshi refuses to discuss the matter further in her presence. Worse, Deputy Ambassador Werther catches them all together and accuses them of trying to subvert the talks; Kothar's loyalty is thus called into question, and he's forced to take a hard-line stance whether he wants to or not.

Frustrated, Benny visits Ithva to discuss her lack of progress, while Sharintha plays with Ixiss, a shield-lizard that her father gave to her as a pet. Ithva reveals that Hass allowed her to access his gardening porters' surveillance records; the footage is corrupt and inconclusive, but it seems that a Mim and the missing Draconian clerk Pashar went into the shed where Lwpha's body was later found, and that Ms Haddad is the only one who came back out. If it comes out that Haddad killed a Mim and a Draconian, then Earth -- which is already drifting in a xenophobic political direction -- could find itself at war. Benny tells Ithva that her research has turned up evidence of something called Project Narcissus, a military research project established during the war with the Daleks; however, she doesn't reveal that she learned of it through secret military codes etched onto a Mim artefact found buried in the ruins of Windsor Safari Park, just a few miles away from her father's home on 20th-century Earth.

Benny tries to smooth things over with Mwshi and Kothar, but fails. Mwshi admits that the few surviving Mim need help to back them up against the Draconians, and that in exchange for Earth's support, they've agreed to hand over about a hundred of their children to the Institute. However, he reacts with shock when Benny reveals that it was Korenthai who used phase cannons against Proxima Longissima, and Benny realises that he knows more than he's saying. Her meeting with Kothar goes even worse, as Kothar has learned about the evidence implicating Ms Haddad, and he threatens to go public with it unless Benny convinces the Mim to cede control of the borogoves to the Draconians. As if Benny didn't have enough to worry about, a Mim then breaks into Peter's bedroom, and Jason barely manages to hold it back until Peter can call in "Daddy Adrian" and his Killoran security guards. The Mim is captured and locked up in a containment field, and Benny, shaken, decides to take Peter back to 1987 to stay with his grandfather Isaac until it's safe. In fact, Benny has another motive for visiting 1987: to learn what her father knows about Project Narcissus.

Isaac explains that, during the war, he was occasionally tasked to deliver "payloads" to occupied planets. He didn't know what the payloads consisted of at the time, but after seeing the other side of the war through the Dalek war computers and meeting Lwpha on Phaaag Zenbrou, he put the pieces together. At some point during the war, the Earth military "acquired" a borogove, and brainwashed and conditioned the Mim children to infiltrate the enemy, programmed with deep-seated hypnotic triggers that caused them to turn on the enemy from within during critical battles. Thousands of Mim children were tortured, killed, and driven mad by Project Narcissus, but they helped to win the war. It's an appalling moral grey area that Isaac wishes he could have protected his daughter from, but even though he feels that it was necessary at the time, that doesn't mean that it was justified -- and it certainly doesn't mean that it should be repeated. And he and Benny both believe that Szmyt and Haddad are trying to revive the project, not against a ruthless enemy like the Daleks, but against anybody they imagine poses a threat to the increasingly xenophobic 27th-century Earth. Shaken, Benny tells her father about the visit from the Quire, posthumans from the distant future with full family names including "Quarterstaff" and "Fallowsolstice"; only after they'd gone did she realise that these could be translated as "cane" and "field of summer." Benny's legacy might last all the way to the posthuman epoch if only she can keep the children of today safe -- but how can she justify acting as though her children have more of a right to survive than anybody else's?

While Benny is away, Mwshi confronts Szmyt and Haddad, and is fired from the Mim diplomatic staff as a result. Ms Haddad goes missing, possibly because she fears that Kothar will indeed expose her as a killer. Jason freezes a sperm sample as insurance and confronts Szmyt, accusing him of torturing alien children for an imaginary greater good; however, Szmyt contemptuously dismisses Jason as a hick colonial with a hybrid half-human child, and informs him that the Institute must act to keep future supplies of Mim children out of the hands of other races who would no doubt use them in Narcissus-type projects of their own. When Benny returns, Jason tells her what's happened... and Benny puts the pieces together and realises the truth. When she interrogates the Mim who supposedly attacks Peter, her suspicions are confirmed; the Mim goes by the name "Victoria", which was given to her by the Institute. The Mim gangster Sherm was offworld during the genocide because he was selling children to the Institute, and Korenthai -- or "George" -- isolated the borogoves on Proxima Longissima on their orders, as they had anticipated trouble due to the war and wanted to set up an alternative supply of Mim children. They're not reviving Project Narcissus; it never stopped.

Victoria reveals that some of the Narcissus agents have shaken off their conditioning; they call themselves the Children of Nemesis, and they've been trying to rescue the borogoves before they fall into the hands of the Institute. Victoria wasn't attacking Peter, she was trying to contact Benny to explain the truth. Victoria first arrived on the Collection in the form of Pashar, and she contacted Lwpha and offered to pool their resources. However, Ms Haddad followed Lwpha to the rendezvous, pretended to be his Mim contact, and killed him to prevent him from interfering in the Institute's plans. Victoria arrived too late to save Lwpha, so she smothered Ms Haddad, disposed of her body via the garden's compost transmat, and began impersonating her. She intended to arrange for the Mim children to be taken away from the Draconians as the Institute wished, and then send them to a place of safety to be cared for by the Children of Nemesis; however, because Jason got her captured, Ms Haddad has gone missing too long to be trusted. Benny thus comes up with a better plan.

Benny and Victoria explain the situation to Mwshi, and Victoria then takes on Ms Haddad's form and convinces Szmyt that the Institute's Director had ordered her off the radar to investigate Szmyt's loyalties. Benny, claiming to hate Kothar for threatening her son and getting Bev Tarrant killed, convinces Szmyt that she has a plan to put one over on the Draconians. Szmyt accepts her plan, and goes to Kothar with Benny and Victoria, who is now posing as Pashar once again. Pashar claims to have gone into hiding after seeing Werther transform into a Mim and kill Lwpha, but Szmyt then speaks a code word that trips Victoria's deep-seated conditioning and causes her to spontaneously revert to her sponge-like Mim form. Victoria escapes, and Szmyt assures the shaken Kothar that he's revealed the truth in order to foster better relations between the humans and Draconians. As Szmyt leaves to co-ordinate the hunt for the "rogue" Mim, Benny tells Kothar that at least 30 high-ranking Draconian officials and noblemen are Mim in disguise, but reveals that they're tired of spying and are ready to make a compromise. The Draconians will be allowed to keep the borogoves until they come of age, at which point the Emperor will grant the Grace of Emancipation and effectively banish them from his empire with honour. In the meantime, the surviving Mim will be given full access to Proxima Longissima to educate their children in the ways of Mim culture... and the Draconian officials appointed to oversee the project will be "selected" from the Children of Nemesis. To the public, it will appear as though the Draconians are in charge, but in fact it will be Mim on every level of the process. And if Kothar refuses to accept this compromise, then the Children of Nemesis are highly-placed enough to bring down the entire Empire from within.