The novel begins a few months earlier than the end of Mission of Honor.
In March 1922 PD, tensions between the Star Empire of Manticore and the Solarian League begin to escalate. Considering itself in a de facto state of war with the League ever since the Battle of Spindle, Manticore begins to enact its emergency plans for an armed conflict with the Solarians.
Codenamed Case Lacoön, this plan consists of two phases. On Phase One, Manticore orders an immediate recall of all its merchant shppng from Solarian space, while on Phase Two the Royal Manticoran Navy deploys several task forces to seize control of every wormhole bridge and junction in range and close them off to Solarian ships. Clashes between Manticoran task groups sent to blockade the wormholes and Solarian forces that try to keep them open take place across the galaxy, with the League being defeated or forced to withdraw in each and every one of them.
As the effects of Manticore's actions begin to reverberate throughout the League, Permanent Senior Undersecretary Innokentiy Kolokoltsov of the Solarian Foreign Ministry and his counterparts in the Interior, Education, Treasury and Commerce departments, who effectively control the League, start feeling the pressure from the transstellar corporations and growing media criticism. While it is clear that Manticore is in a position to inflict tremendous damage to the League economy by strangling its trade and cutting the shipping tariffs upon which most of its government is funded, the "Five Mandarins" (as the Solarian press nicknames them) are not willing to pay the political and diplomatic price of taking responsibility for the actions of the League Navy, and continue to believe in the League Navy's technological superiority over the mere "neobarbs" of the Star Empire.
The five Mandarins come to support the plans that the Chief of Naval Operations of the Solarian League Navy, Fleet Admiral Rajampet Rajani, has been pushing for an invasion of the Manticore System itself, despite the risk that failure would make them look even weaker. To support the main attack force of over four hundred superdreadnoughts commanded by Fleet Admiral Massimo Filareta, which is headed towards Manticore via hyperspace, the Mandarins decide to send an additional one hundred superdreadnoughts through the Beowulf Terminus. However, the Beowulfan government refuses to comply with the League's initial "request", stating that under the League Constitution it has the right to do so unless the League has declared a state of war.
In the meantime (and catching up with the plot as it concluded on Mission of Honor), Captain Anton Zilwicki and Agent Victor Cachat arrive in the Haven System and present their evidence of Mesan involvement in the assassinations that triggered the resumption of hostilities between Haven and Manticore to President Eloise Pritchart. A Mesan defector, Dr. Herlander Simões, offers information on the new nanotechnology used to turn people into unwitting assassins. Pritchart then sets out for an unexpected state visit to Manticore and presents this evidence to Queen Elizabeth III herself, who is finally convinced that Haven is no longer the enemy and agrees not only to negotiate a peace treaty, but to accept Pritchart's offer of a military alliance against the upcoming League attack.
Hearing about this visit and the intelligence Zilwicki and Cachat brought with them, representatives from the Beowulf government arrive to join what will become known as "Grand Alliance". Shortly afterwards, Protector Benjamin IX arrives from Grayson to participate in the peace talks and to represent the other original members of the Manticoran Alliance.
Even more unexpectedly, Honor Harrington is approached by a treecat memory singer called Sorrow Singer, who is the only surviving member of the Black Rock Clan, which was wiped out by the Mesan Alignment's Operation Oyster Bay. The alien offers the humans her species' assistance in guarding against the new assassination technology that was used by Mesa and to help them in their security, as their empathic sense allows them to perceive malign intentions and detect the instant when a Mesan nanotech victim is triggered.
Grand Admiral Chien-lu Anderman, the cousin of the Andermani Emperor and a close acquaintance of Honor during her past deployments to the Silesian Confederacy, arrives at Manticore to take stock of the situation. Honor and Queen Elizabeth convince him that the best possible course of action for the Andermani Empire is to declare its neutrality in the current conflict with the Solarian League, which will grant it full freedom of action to go against the Mesan Alignment if Manticore and her allies find themselves unable to do so.
To the surprise of the entire human-settled galaxy, Haven and the Manticoran Alliance sign a peace treaty and reveal what they know about the Mesan Alignment, its plans and intentions, and its capabilities. On Mesa, Albrecht Detweiler reacts with a more than justified Precision F-Strike to these news, which effectively shatter centuries of covert action aimed at imposing a new galactic order based on genetic engineering. Since the Star Empire of Manticore's Talbott Quadrant is far too close to Mesa for comfort and a direct attack might be imminent, Detweiler activates "Operation Houdini": the covert evacuation of as many of the Alignment's members and officials from Mesa as quickly as it can be arranged without revealing even more about it.
After the signing of the peace treaty and the failure of some last-minute attempts to convince Kolokoltsov and the other Mandarins to cancel their attack on Manticore, the leaders of the Grand Alliance begin planning for the expected arrival of Admiral Filareta's Eleventh Fleet, and decide that simply deterring his forces would not be enough in the long term, as the Mandarins would find some way to spin the affair in their favor. Hamish Alexander makes the point that they need to create a situation that is an "unambiguous, undeniable, decisive defeat" for the Solarian League Navy, while Honor adds that the Sollies must be made to understand that a confrontation with Manticore is war and "that wars have consequences".
Admiral Filareta finally arrives at Manticore with his massive fleet of four hundred and twenty-seven superdreadnoughts, and proceeds upon a course to Sphinx, Honor's homeworld, in a maneuver intended to give him sufficient time to assess the odds and pull out back into hyper if necessary. A "Grand Fleet" of Mantícoran, Havenite and Grayson warships, commanded by Honor, executes a plan of misdirection, making Filareta believe that Manticore made the mistake of splitting its forces to defend both Manticore and Sphinx. As the two sides approach one another, Manticoran light attack craft take out Filareta's advance recon platforms. At the Wormhole Junction, a Solarian journalist dispatch boat is allowed transit, despite the fact that Manticore knows it is really under the control of the Solarian Navy and meant to signal the Solarian forces in the Sigma Draconis System that Filareta has arrived.
Honor then contacts Filareta and attempts to dissuade him from continuing on his mission. Confident in what he believes to be the overall superiority of his forces, the Solarian admiral refuses to back down despite Harrington's repeated warnings not to cross the system's hyper-limit, and demands that she stand down her ships and surrender. The Solarian fleet crosses the hyper limit, and Honor springs her trap: additional Manticoran forces appear on Filareta's plot as they come out of stealth, making Harrington's forces climb from forty to two hundred superdreadnoughts and moments later the Havenite force drops out of hyperspace behind Eleventh Fleet with two hundred and fifty additional superdreadnoughts and LAC carriers.
Harrington then contacts Filareta again, this time using faster-than-light communication, thus rubbing in Manticore's technological superiority. Thomas Theisman, Haven's secretary of war, appears beside her on her flag bridge; they reveal the presence of a quarter-million missile pods ready to rain destruction upon his fleet. Filareta realizes that his position is hopeless and agrees to surrender, but to his shock, his operations officer suddenly opens fire with the fleet's missile pods and then enters a code that detonates a bomb, placed there by Mesa for this case, killing Filareta and his entire flag bridge crew. Unable to believe Filareta's apparent reaction, Honor promptly opens fire in response to the attack.
In the Sigma Draconis System, Fleet Admiral Imogene Tsang's task force receives the activation order from the dispatch boat, and proceeds towards the terminus, only to be confronted with the Beowulf System-Defense Force, which refuses to permit Tsang to transit through the terminus and tells her that any attempt to seize control of it will be met with deadly force. When Tsang refuses to back down, sixty Manticoran superdreadnoughts come out of stealth, eliminating Tsang's 3-to-1 advantage over Beowulf's ships. After a heated exchange, Tsang backs off and leaves the system.
The total losses for the Solarian League in the Second Battle of Manticore are staggering: almost three hundred superdreadnoughts destroyed outright or damaged irreparably, another seventy ships damaged and over 1.2 million dead Solarian personnel. Manticore's own losses are marginal with the loss of mostly LACs and "only" 2,000 dead. Honor blames herself for all losses of the battle, believing she pushed Filareta too far. However, her fellow admirals point out that Filareta's missile launch was too uncoordinated, and concludes correctly that this was another example of Mesa's nanotech, since a coordinated launch would require far more complex orders than for someone to simply punch one button.
Upon Admiral Tsang's return to the Sol System and the news of the Eleventh Fleet's destruction, the Mandarins find themselves forced to contend with the consequences of the debacle. Their failures are likely to bring about the collapse of the League, as the media and citizens are taking a closer look at the constitution and start to question the bureaucracies' ability to make the gigantic nation work. The Mandarins arrange for the Solarian government to accuse Manticore of perfidy, claiming that Honor fired on Eleventh Fleet after Admiral Filareta had surrendered, and to charge Beowulf with treason and collusion with a hostile power, in spite of being one of the League's oldest and most prestigious members, banking upon the public's unwillingness to believe that the League could be defeated so handily by a "neobarb" star nation. The Mandarins' efforts are somewhat successful and the Manticoran embassy and the Beowulf Assembly Delegation Office are besieged by angry mobs who protest their actions.
Meanwhile, Queen Elizabeth helps organize the marriage of the heir apparent, Prince Roger, to which much of Haven's political leaders are invited, in addition to the Grayson, Beowulfan and other foreign dignataries already present on Manticore. President Pritchart receives word from Haven that the peace treaty has been ratified by the Senate and that the Second Havenite-Manticoran War is officially over.
Under the auspices of the new "Grand Alliance", Manticore sends all the research and development staff that survived the destruction of the space station Weyland to Haven's secret Bolthole facility to proceed with a joint weapons development program, while Beowulf provides financial and technical assistance for the reconstruction of Manticore's orbital stations and space-based industry. The new relationship with Haven and Beowulf allows the Star Empire to compensate for the loss of the Solarian markets and to vastly reduce the time needed to rebuild from the catastrophic Mesan attack.
On Old Earth, Admiral Rajampet does not intend to become the Mandarins' scapegoat and prepares to release damaging information on all of them should they try to lay the blame of this failure upon him. But before his meeting with them, he suddenly commits suicide, yet another victim of Mesan nanotech.
The death of Admiral Rajampet spurs a pair of iIntelligence analysts from the two main branches of the Solarian League Navynote , Captains Daud al-Fanudahi and Irene Teague, to arrange a clandestine meeting with like-minded counterparts in the intelligence branches of the Solarian Marine Corps and Gendarmerie. For quite some time, al-Fanudahi and Teague have suspected that there is actual merit in Manticore's claims that the League is being manipulated by someone from the outside intent upon its destruction, and the sudden death of Rajampet after having spurred the SLN to what ended up as its worst defeat all but confirms their suspicions. The Marine and Gendarmerie intelligence officers are convinced, and the four of them agree to quietly and covertly look into such outside influence.
The late Admiral Rajampet's deputy, Admiral Winston Kingsford, becomes the acting head of the SLN and is summoned to Kolokoltsov's office to brief him on the League's military options regarding Manticore. Kingsford promptly informs Kolokoltsov of the unpalatable truth that Manticore's military advantage over the League is so overwhelming that any further confrontations between them would be one-sided massacres similar to the Second Battle of Manticore.
The economic and political aspects of the League's predicament are even worse. According to an analysis by the SLN's Office of Naval Intelligence, the League economy, which has already taken a severe blow due to Manticore's recent actions, is likely to be slow to recover, and the total economy of the Grand Alliance is likely to match the League's within a decade or so. That, of course, assuming that the League can survive the increasing risk of uprisings in its "Protectorates" now that the illusion of invincibility of the League Navy is gone for good, the growing public questioning of just how dysfunctional its government has proven to be, and the possibility of some of its member worlds opting to secede altogether. Ultimately, Kingsford presents Kolokoltsov with their only options: fighting a commerce-raiding war until they can match Manticore's military capability, or sue for terms… if enough of the Solarian League can survive that long.
Kolokoltsov convinces the other Mandarins that their best chance to stay in power is to adopt Kingsford's recommendations, score some victories on the Manticorans and only then attempt a negotiated settlement that would not be detrimental to the League's internal stability. They also maneuver to have the Beowulfan government investigated for treason as a motion to that effect is carried within the League Assembly with over three-quarters majority... but there are enough dissenting votes to instill a sense of doom over the League's unity in the short term. The maneuver backfires badly however, when the Beowulf delegation declares its intention to invoke a never-before-used section of the League constitution which allows member systems to secede from the League, to be confirmed by a referendum of Beowulfan voters.
As Beowulf prepares for the plebiscite vote, which is all but certain to pass by a huge majority, Manticore provides technical assistance to reinforce its defenses, but is altogether prevented from stationing warships in Beowulf to avoid any appearance of coercion. The Mandarins, still trying to hold the federal system together, see Beowulf's impending secession as a serious threat, not only to the League's cohesion but to its security as well and begin planning towards addressing that threat.