Yeah so MAJOR SPOILER ALERTS, obviously.
A UNSC Science team under ONI finds the damaged remains of an old Forerunner monitor, and starts interrogating it about the Didact.
The monitor begins his life story, occasionally breaking through the UNSC firewalls as if they were made of paper, in order to access old Earth records. The ship’s captain, and ONI, are not thrilled about the security breaches. But the science team continues interrogating.
The monitor says he used to be the ancient human named Chakas, from the first forerunner book Cryptum. He starts from where we lost Chakas in the previous book, when the Master Builder kidnapped them all. Chakas was stuffed into a life pod, then ejected during the battle of the forerunner capital from the first book.
He crash landed on a Halo ring, in the middle of some field, and was found by humans who lived on the ring.
Then the vast majority of the book talks about Chakas and his new human friends walking around the ring with almost no apparent goal in mind. Boring. Skip this.
Chakas discovers that he is host to an ancient “spirit” personality that observes the world through his eyes and can speak to him through his thoughts. Chakas thinks this is magic. In reality, it’s a by-product of forerunner genetic memory/personality storage.
The personality is that of an ancient human named Forthencho, the Lord of Admirals. He was defeated long ago by the Didact, his sworn enemy, during the human-forerunner war. Instead of killing him after his defeat, the forerunners decided to preserve his memory and personality into future human generations because they thought that he and the other human commanders of the war knew the only non-Halo solution to the Flood, since the humans had fought it off before and won, as mentioned in the previous book, Cryptum.
Chakas wanders the ring with his friends some more, while the Lord of Admirals makes remarks in his head like a backseat driver. He eventually finds his old friend Riser from the previous book, who also has the genetic imprint of an ancient female human commander with a ridiculous name I can’t remember.
Chakas and Riser’s imprint personalities kind of have a proxy argument through their hosts, since they were old rivals who blamed each other for losing the human-forerunner war. Whatever, continue…
Eventually Chakas and Riser get captured by the ever-famous forerunner AI Mendicant Bias, who is in control of the ring and has been running Flood-infection experiments on the humans living on the ring for some time. He extracts the Lord of Admirals’ personality and puts him into a monitor, which makes things slightly less complicated for Chakas. Lord of Admirals proposes to seize control of the Halo and turn it against the forerunners, to which Mendicant agrees, since they both hate the forerunners obviously.
Mendicant Bias gives Chakas and a number of other humans a proposition. He would temporarily grant control of certain Halo systems to the humans, because currently the Halo is on a collision course with a nearby planet, which is a problem. They will help to save the Halo, or die in the otherwise inevitable collision.
Bias plugs the humans into the ring’s control systems via the ring’s Silent Cartographer, which is currently the only structure not sabotaged by the remaining forerunners on the ring. They deploy hardlight spokes to attempt to absorb the planet’s kinetic energy, but since most power stations were sabotaged by forerunners, many sections are still damaged and require self-repair. Parts replacements from the Greater Ark and Lesser Ark (yes, there are two of them) are impossible since they are under forerunner control.
Then the new Didact, the Bornstellar incarnation, appears with a forerunner armada and seize control of the ring from Mendicant and his friend the Primordial, aka the Captive.
Chakas is gravely injured during the process somehow, I forgot since the book didn’t make it very memorable, and he is saved by the New Didact by being placed in a life support system. Together, he and the Didact interrogate the Primordial. The Primordial reveals that the forerunners were supposed to be exterminated by the Precursors after they were created, and that humans were supposed to be the next species to be tested. But the forerunner rebellion against their creators long ago screwed that process up.
The connection between the Precursors and the Flood isn’t very clearly established. The Primordial claims to be the last precursor, but physically he is obviously a Flood Gravemind. Didact is frustrated by the lack of real answers, and kills the Primordial.
The ring, in the meantime, sheds its damaged sections, including life support. It becomes entirely covered in clouds as a result. It is transported to the Greater Ark, and then reassigned to the Halo Array as Installation 07.
Chakas’ physical body is too damaged to survive normally, so the Didact offers to turn him into a monitor. It happens, and Chakas assumes a new form, as 343 Guilty Spark.
Spark, in the meantime, finishes his story to the ONI interrogators. The humans debate the validity of his story, since some things contradict current known forerunner facts, but others fit quite conveniently. Spark loses power and shuts down, but the ship’s AI announces that it has released control and granted Spark full access. He apparently downloaded himself into the ship while they weren’t looking.
Spark alters the atmosphere of the ship sufficiently to put the crew to sleep, and announces his intention to commandeer the ship to find the Librarian, since he claims that she is still alive, and that he knows where to find her.
EDIT: Wow, that was a lot longer than I expected. :fffuuu: