From Halopedia, the Halo wiki
Halo: Retribution is a novel by Troy Denning. Having first appeared on Amazon's website, Retribution was officially announced on June 23, 2017, and was released on August 29, 2017. Set in late 2553, the novel follows up on Denning's Halo: Fractures short story A Necessary Truth, although many of the central characters from the preceding novel Halo: Last Light also make a return.
In December 2553, Veta Lopis and her Ferret team—comprised of the SPARTAN-IIIs Ash-G099, Olivia-G291, and Mark-G313—are on a mission to find the abducted family of UNSC Fleet Admiral Graselyn Tuwa, who was recently assassinated. While disguised, the team gain the trust of a group of Kig-Yar in league with the Keepers of the One Freedom, their primary suspect. While aboard the Kig-Yar's ship, which is heading to the Keepers' hidden Salvation Base, the Kig-Yar attempt to gas them, but the Ferrets take over the ship for themselves, posing as "believers." Meanwhile, SPARTAN-II Blue Team tracks the Ferrets from aboard the UNSC Silent Joe, one of the Office of Naval Intelligence's Sahara-class heavy prowlers, as backup. Eventually, the Ferrets infiltrate Salvation Base—on a strange Forerunner world—in search of the Tuwa family.
While at the base, Olivia-G291 discovers a Turaco ship with evidence of a connection to Dark Moon Enterprises, which planted the bodies of the Tuwa family within the detention center of Salvation Base to frame the Keepers of the One Freedom. A battle occurs when the Ferrets cross paths with the Keepers' leader, Castor, but Lopis and the Ferrets escape in the Turaco with Blue Team as Salvation Base is destroyed.
Castor, having also discovered the Tuwa's corpses, decides to search for the Dark Moon operatives as well. Although not working together with Lopis, they both follow the trail of evidence back to a laboratory on Gao, where President Arlo Casille admits to having made dealings with Dark Moon. Casille allowed them to use his labs if they promised to deal with the Keepers, who wanted vengeance on Casille after the recent conflict on the planet. Dark Moon had previously attempted to manufacture a harmful bioweapon researched on Argent Moon using the Tuwa's remains, but they succeeded within the labs on Gao.
After interrogating Casille, the Lopis and her Ferrets were led to Pinnacle Station above Meridian, where the AI Governor Sloan had initially advised Casille to work with Dark Moon Enterprises. The Keepers, the Ferrets, and Dark Moon with their bioweapon all converge on Pinnacle Station and a battle erupts, eventually traveling to the surface of Meridian. Although the Spartans and Ferrets manage to neutralize the bioweapon and Dark Moon, Castor escapes.
After the narrative ends, it is revealed that the Forerunner Ancilla, Intrepid Eye, was behind the bioweapon plot and Dark Moon Enterprises. With her plot at least temporarily foiled, Intrepid Eye framed Bartalan Craddog with creating the bioweapon and remained secretly in control of the station and further secret projects. The Spartans and Ferrets never learn this truth, and the true mastermind of the plot is only revealed to the reader.