Summary: Prologue: Ben's sister Judith confides to Kassidy that her father Joseph won't come out of his bedroom; the loss of Ben and Jake is too much for him. Kassidy says she might know someone who can help. Shortly thereafter, the O'Briens and their 2 children arrive.
The mission to the Gamma Quadrant is 90% complete, and the crew of the Defiant are ready to be back home, especially since their encounter with the cathedral cost them the use of their replicators. One of their probes detects a Starfleet transponder signal on a narrow subspace band, which shouldn't be possible, since Starfleet has supposedly never been in the area. Dax takes Bowers' report to Vaughn, and he is incredulous; he immediately tells Dax to cloak the ship, get the Sagan shuttle back at 100% ASAP, and to not tell the crew. He recognizes the code as that of the Valkyrie.
Asarem is sworn in as the First Minister of Bajor, after Shakaar was assassinated by Hiziki Gard, the aide to the Trill ambassador.
The Defiant arrives at the planet Uridi'si, where the transport signal originated from. They find the wreckage of a Jem'Hadar ship--well beyond Dominion space. Vaughn briefs the senior staff for an away mission to check the wreckage, and tells them to keep the details from the crew.
During questioning, the Trill Ambassaor says that Gard was assigned to him from the Trill Diplomatic Corps out of the blue. Ro finds the assassin's actions strange; unless Gard is a rare, mismatched Trill, the murder couldn't have been a rogue action, but one with purpose.
First Minister Asarem convenes a meeting to say she's leaving DS9 to address the Bajoran people: she cannot accept the Federation's invitation to join at this time, and she will reevaluate the need for Starfleet on DS9. Ro says the assassin may still be on board, but a sensor sweep detects an energy trail consistent with a cloak heading towards the Trill system. Akaar assigns Kira--who, unbeknownst to her is still under commission as a Starfleet officer--to the Gryphon to track the ship, leaving Bajoran General Lenaris in charge of DS9 to work with Akaar himself. Asarem confides with Kira that she's unsure which path to take, since they're the only ones who knew Shakaar's true intentions.
The away team finds the crashed Jem'Hadar ship; scans indicate it's been there about 2 years. There's a lifesign inside, but it ends up being a bipedal bird. Inside are only dead Jem'Hadar and a Vorta; however, Nog feels that they're being watched. Suddenly their scans show a large sauropod nearby, and they realize it's likely a changeling from the crashed ship. They lure it into a trap, causing a phaser to fire, rendering the changeling into it's liquid form briefly, before it flies away. Vaughn signals them to come to his position immediately; he found a crashed Borg ship.
Taran'atar tells Ro that he was monitoring communications and noticed an echo, indicating there was someone else doing the same. She scans for places he could've been hacking the comms, and zeroes in on a location. Shortly thereafter, the power goes out in the wardroom where Akaar and Lenaris are conferring, and Ro beams in with a phaser in hand.
Nog inspects the Borg ship and realizes it was originally a Starfleet vessel; Vaughn says it was the USS Valkyrie. Inside they find 1 surviving Borg that is the source of the signal. Nog's scans indicate that there's no subspace signal being sent to the Collective; Bashir's scans find the lone drone is a female being kept alive only by the alcove. Vaughn tells them to beam the drone back to the Defiant and remove its Borg implants. He tells Bashir the drone's medical records can be found in a subsection of Ensign Tenmei's file--it's her mother.
Ro says the killer is above the main console in ops. Akaar calls for Ro to be arrested, but Lenaris says he'll grant her some leeway as the station's security officer. She fires a phaser into the alcove above, but no one is there; however shortly thereafter, a large dent appears in the table below, and Gard unshrouds. He's wearing a Federation isolation suit.
Vaughn knew the signal was Ruriko's immediately, because special ops transponders are wetwired into the operative's nervous system, and only cease sending the signal upon brain death. Bashir says she is in a coma, and that he can't remove the remaining Borg components until they return to DS9; Vaughn says he'll do it now, as he won't put the station at risk. Prynn walks in and Vaughn screams at her to get out, confining her to quarters. Bowers tells Dax that Vaughn is losing it; she says she'll talk to him.
The doctor finishes Shakaar's autopsy and reports that there was isoboramine in his blood--the chemical found in joined Trills--and symbiont DNA in Shakaar's wound. Akaar immediately recognizes this as one of the parasites that infiltrated Starfleet 12 years ago.
Akaar interrogates Gard, who tells him the parasites are essentially the same species as Trill. Shakaar had been infected for months. When Akaar asks about the cloaked ship, Gard is shocked, knowing nothing about it. He says Captain Mello of the Gryphon must also be infected and is heading to Trill to retaliate.
O'Brien tries to talk to Joseph Sisko, but he tells him to get the hell out of his bedroom and to mind his own business. O'Brien intentionally boils up some extra-stinky corned beef and cabbage, which offends Joseph's senses so much that he comes out to make the O'Briens a proper meal.
Captain Mello tells Kira she needs to sleep. First Officer Montenegro takes her to a luxury sweet, but she asks for something a little more utilitarian.
Dax confronts Vaughn about how he's putting his wife ahead of the crew, and though he screams at her at first, eventually he breaks down and tells her everything.
20+ years before, Vaughn is captured on a Cardassian planet while trying to help a renowned cybernetisist, Dr. Veruda, escape. The interrogator, a Cardassian woman named Kree Omiturin, helps him escape with the doctor in tow. He later finds out that she was his biggest rival in Starfleet Intelligence--Ruriko Tenmei. He is in love with her.
His Vulcan counterpart T'Prynn is killed in a transporter accident. Ruriko comes with him to the funeral on Vulcan, and they find they're still holding hands long after the ceremony.
Dr. Veruda's AI successfully neutralizes a Cardassian shipyard. Ruriko says sure wants to step away from SI for a few years... so she can have a child with Vaughn.
Vaughn gets called away on Prynn's 5th birthday, after having been gone for months already. Another mission prevents him from congratulating her on her entrance into Starfleet.
Ruriko and Vaughn finds themselves at the planet Uridi'si, where Dr. Veruda's AI is once again active, this time in the planet's magnetosphere. A Borg vessel pops out of a transwarp conduit nearby, apparently summoned by the AI. After disabling Vaughn's ship and chasing off the Valkyrie, it downloads the AI into it's system. Realizing there's no way they can let the Borg leave with control of the powerful AI virus, Ruriko and her team been aboard the Borg ship to deploy a nanite weapon they developed to neutralize the virus. The Borg ship enters into the transwarp conduit, and the Valkyrie follows.
Vaughn tells Dax he knows he made the wrong decision, but that for once in his life, he put himself first to save his wife. She says it's understandable, given the astronomical circumstances, and he says there's a good explanation for it.
Quark visits Treir in her quarters; Morn is just leaving. Quark tells her about his plans to run away with Ro, but thinks she's changed her mind since the Shakaar incident. Treir says that if he really loves her, he'll be happy no matter what she decides. They hear a noise in the hall and find Gul Macet clawing bloody streaks in his face, with smoke coming from his mouth, and a blue thorn-like creature wriggling in his neck.
Captain Mello pushes the warp engines too hard and an EPS conduit ruptures. The chief engineer and some other are badly injured, but the warp core doesn't fail. Montenegro tells Kira that the Captain has changed and that he doesn't know her anymore; she feels the same way about Shakaar, but tells him that he'll have to address his feelings with the captain. Later that night, Kira gets an encrypted transmission from Akaar detailing the parasite they found in Shakaar, and warning her that there is no cloaked ship. She remembers that Shakaar spent most of his time with Captain Mello, and goes to find Montenegro.
Vauhgn confronts Prynn; she has been surruptitiously modifying the sensor arrays to search for her mother's transponder signal on every ship she's been on since her mom disappeared. Vaughn thinks the whole reason they found her was because of the Orb. He takes her to see her mom, whose human physiology has begun reasserting itself thanks to Bashir's treatment. Prynn hums her mother's favorite song, and her eyes open; her mouth seems to be trying to say "Prynn".
Kira convenes a meeting with Montenegro and Dr. Xiang. The doctor doesn't believe the captain is compromised, but Montenegro found the program used to fake the cloaking-device reading that was uploaded from the captain's quarters. They plan to sedate the captain, turn the ship back towards DS9, and remove the parasite, if possible. They go to the captain's quarters and hold her at phaser point, but when the doctor checks her neck, there is no blue spike there. Montegro turns his phaser on Kira, locks them in the captain's quarters, and initiates a program.
Shar beams back down to the Borg ship to retrieve another Borg drone, so he can access its neuroprocessor to find out what its mission to the Gamma Quadrant was; Commander Tenmei's was destroyed when she was damaged. Nog and Bowers try to find the changeling so they can convince it to return with them. They find it back in the Jem'Hadar ship, and surprisingly they do not have trouble convincing her to come back with them at all.
Shar finds that the assimilated Valkyrie's mission was to find species desirable for assimilation as a prelude to a larger-scale incursion. When the Borg attacked the Federation 3 years prior, they learned of the Dominion and erected a transwarp conduit to the Gamma Quadrant, the end result of which was the crashed Jem'Hadar and Borg ships. They hear a scream and find the Borg corpse has come to life and is attempting to assimilate the changeling. It changes shape and color and shrieks wildly, but eventually reverts to humanoid form, having crushed the nanoprobes. They realize the Borg can still attempt to assimilate other life-forms even after the death of the drone's organic half. Realizing Prynn could be next, he rushes in, tells her to step back, and fires his phaser at Ruriko.
An alert blares that the Gryphon's warp core will breach in 2 minutes; Montenegro tells the crew to abandon ship. Mello tries to contact the bridge, but Kira says Montenegro has them locked down, explaining the entire situation. They see the escape pods being jittisoned, but when the countdown hits 0, the ship continues on towards Trill. Later, some crew that had remained aboard, certain there was nothing wrong with the warp core, free the captain from her quarters. They quickly formulate a plan to take back the ship. They arm themselves and easily break onto the bridge, but it's empty, control having been re-routed to engineering.
Kira and Mello try to enter engineering via Jeffries tube, but Montenegro fires a phaser at them, burning a hole through the captain's chest. Before she dies, she transfers control of the ship to Kira and hands her her combadge.
Joseph Sisko comes around to enjoying the O'Brien's presence quickly, especially Kirayoshi and Molly. Judith reminds him that Kassidy is due for a little one of her own, and is alone on Bajor. The next morning, he asks the O'Briens to take him there.
Dax tells Prynn that Dr. Bashir has ruled that the damage to Ruriko's brain was too extensive to bring her back, but she is still furious that her dad killed her mom a second time, even if she was about to assimilate Prynn. Vaughn later asks Dax why she lied to Prynn; she says because Prynn won't understand that he killed Ruriko so she didn't have to live with the horror of trying to assimilate her own child.
Kira finds Montenegro in engineering. He tells her that they plan to enter the Celestial Temple, and that the Trill are more dangerous than his kind are. They fight briefly, but he easily gets the upper hand. As he's about to kill her, she pulls Captain Mello's phaser from her boot and blasts him in the face. A six-legged creature with pincers crawls from Montenegro's mouth, and she stomps it to death. Kira transfers control back to the bridge, but 3 Federation ships arrive and begin attacking them. With the communications array slagged, Kira says to use the phasers to spell out "Manufactured triumph", hoping that the pursuing Captain Solok will remember using the phrase when Kira played baseball against the team of Vulcans on the holodeck a few years ago. With their shields gone, Solok and a team of Vulcan security officers beam aboard to offer assistance.
The Gryphon is restored, and the escape pods recovered. A Trill military transport arrives, with General Taulin Cyl of the Trill Defense Ministry aboard. He says they need to discuss the parasite's coming war of revenge against the Trill symbionts; humanoids aren't the targets, the Trill are.
Epilogue: An hour from the wormhole, they find that the subspace communication relay they left there has been destroyed, and that the space around the wormhole has been altered: the wormhole now opens at the Idran system, the whole of which has also moved. A Jem'Hadar ship approaches and hails them; it's the Vorta Weyoun, and he demands they return the Founder they have on board. Vaughn agrees, and says they would also like to arrange an information-exchange pact on the Borg; Weyoun says he'll pass it on to the Founders. The changeling tells Weyoun to thank the Federation for saving her life; Weyoun says as a thanks, they have an exchange for them as well. He beams 3 people aboard: Jake Sisko; a female named Wex, who Jake says is on a pilgrimage; and former kai of Bajor, Opaka.
Review: This book was so much shorter and faster-paced than the other 3 in the series; it was action-packed from start to finish. It brought back some unexpected old foes in the Borg and the parasites; Gul Macet is seemingly dead; Jake is back; we learned a lot more about Vaughn's backstory; etc. 5 stars.