From the introduction:
The PCs begin mid-mission, exploring a section of the mysterious false moon of Salvation’s End, when a group serving the mysterious Vault Lord appears and attacks! After the PCs defeat these adversaries, several drones enter the area and relay instructions from the guiding intelligence of Salvation’s End. The intelligence briefs the Starfinders on another vault under attack and through reason, intimidation, or appeal to their compassion for the test subjects, insists the Starfinders help it stop the Vault Lord.
The intelligence directs the PCs to a vault simulating Absalom Station during the Magefire Assault that occurred over 300 years ago. Arriving at the first access point, the PCs must contend with a large force of undead. Conditions become increasingly hostile. The Starfinders must rescue civilians in danger and overcome traps, hazards, and a shard of unliving magefire.
Eventually, the PCs reach the false Starstone Reactor chamber of the station, where the Vault Lord’s servants perform the steps necessary to seize the Absalom Station vault. A holographic representation of the Vault Lord appears and converses with the Starfinders. Realizing the PCs are working with the moon’s intelligence, the Vault Lord orders their minions to destroy the Starfinders and animates the surrounding debris and reactor components into an immense construct to annihilate the PCs and seize the vault!