Sergeant Borhus and his Deathwatch kill-team are sent to claim a dangerous xenos artefact from their Adeptus Mechanicus allies, to be quarantined by the Inquisition. But a sub-sect of the Cult Mechanicus, headed by the deadly Electro-Priests, venerates the artefact as an example of the Machine God's will, and will stop at nothing to retrieve it. Borhus and his Space Marines face a battle they didn't expect and couldn't possibly have prepared for – and may not survive.