Plot
The game's story, written by author Dan Abnett, focuses on a squad of Inquisitorial Agents investigating a potential Chaos infiltration in the Hive City of Tertium on the industrial planet of Atoma Prime.
The player character Operative begins the story detained aboard the prison transport Tancred Bastion for a crime determined during player creation. During transport the Tancred Bastion is suddenly swarmed and overwhelmed by followers of the Chaos god Nurgle. The player rescues Explicator Zola and the two successfully evacuate the ship. Zola agrees to spare the player from the death penalty by conscripting them to the Inquisition. From their base aboard the ship Mourningstar, the Inquisition deploys the Operative to different sectors within Tertium to research and suppress the Nurgle infestation and cult.
Once onboard the ship, the player begins to earn the trust of the Mourningstar's different members, all of whom are subservient to Inquisitor Grendyl. They must enter the depths of Atoma's largest hive, Tertium, and foil the plans of the chaos cult by claiming supply drops, investigating the daemonic plague, assassinating leaders of the renegade Planetary Defense Forces and reclaiming important facilities across the city. After some time, it is discovered that the Mourningstar has been infiltrated by a spy for the cult, who was sending out detailed reports of the Inquisition's operations and subsequently killing off entire squads of elite agents planetside. Interrogator Rannick, the right hand man of Grendyl, leads the investigation of the Mourningstar's mole problem while also showing interest in the player's progress. Once enough trust is earned, Rannick welcomes the Player fully into the Inquisition while also killing the traitor once and for all. After this, the player continues their ongoing mission to purge the corruption of Nurgle in Tertium as a full member of the warband
The plot is set to evolve over time in a manner that parallels a live service, continuing to develop on a weekly basis and following a set meta narrative.