Story so far

Murderbot is a rogue SecUnit, a part-organic, part-machine security construct that hacked its own governor module and has chosen its own actions ever since. It saved Dr. Ayda Mensah and her PreservationAux team from the mining corporation GrayCris, then refused the home she offered. Alone, it fell in with Perihelion, the sarcastic university transport it calls ART, which rebuilt its body to pass as an augmented human and became the closest thing it has to a friend. It broke GrayCris at Milu, fought Mensah free at TranRollinHyfa, and settled on Preservation Station as a legal refugee. Then ART, seized by a hybrid alien-human intelligence, dragged Murderbot and Mensah's daughter Amena to a lost colony outside the Corporation Rim, where contamination out of a Pre-Corporation Rim ruin was turning colonists into Targets and machines into weapons. Murderbot freed ART and made a copy of itself, Murderbot 2.0, which liberated a Barish-Estranza SecUnit now called Three, then died so Murderbot could destroy the alien network's core. Mensah reached them before corporate reinforcements could finish the job, and Murderbot accepted a trial mission with ART while keeping Preservation as its first home.

System Collapse opens in the middle of an unfinished mess. Murderbot narrates from a moment it will not explain yet, walking a possibly contaminated Pre-Corporation Rim site in an environmental suit instead of armor, then backs up forty-seven and a half hours. Nobody has left the system. Phase I of Plan A: Get the Hell Out of Here is rebuilding the colonists' ancient MedUnits so ART's decontamination package can be run on every human. Phase II is Pin-Lee's legal case against Barish-Estranza's claim of salvage-right ownership of the colonists, backed by a contamination assessment that must certify the site as sealable, or the planet goes under interdict and the colonists have to leave anyway. Asking the colonists what they want founders on whom to ask: they have splintered into more factions than when ART's people arrived. Four days after Mensah's Preservation responder came looking for them, a second Barish-Estranza explorer arrived with at least three more SecUnits, and the corporate task group has been courting the colonists ever since. ART, passing as an ordinary university transport, faced the new explorer down by dropping a weapon port and remarking how easy it is for ships to disappear out here, while Pin-Lee and Turi prepare a counter-bill. And something happened to Murderbot that it will only call redacted; it has been working below its own standards since, and it, ART, Mensah, Seth, and Martyn all agreed it would not go back down to the planet.

It went down anyway. Murderbot was rewatching Sanctuary Moon, nominally supervising Three, when ART broke in: a nine-meter agricultural bot full of contamination had rammed a limb through a hatch and pinned Ratthi and ART's crew members Iris and Tarik in a maintenance bay at an outlying feed router. ART could not use a weaponized pathfinder that close to humans, so Seth handed over a recall beacon launcher repurposed as a bot-buster, apologizing awkwardly for asking. The launcher was too slow to use as designed, so Murderbot crossed a field of tall crops under cover of the wind, shouted the bot off the hatch, and took the tips off two of its legs — but the contamination had augmented its processing enough to read the trap, and it jumped ten meters clear. It was coming down on top of Murderbot when scatter projectiles blew its processor apart, missing Murderbot by two-tenths of a degree. The shooter was a Barish-Estranza SecUnit. Unarmored, Murderbot sent its drones away, locked down its feed, ran its move-like-a-human code, and faked a twisted ankle on Ratthi's arm while five corporates walked up under Sub-Supervisor Dellcourt, carrying weapons built for hunting contaminated bots. Iris traded barbed courtesies with Dellcourt about who would bill whom, then deliberately thanked the SecUnit out loud and ignored his startled objection.

Murderbot is on open ground on a planet it swore never to set foot on again, leaning on Ratthi's arm a few meters from a corporate SecUnit that only has to file a report. Dellcourt's people keep turning up wherever ART's crew works, better equipped for contaminated machines than ART currently is. ART is monitoring every input Murderbot sends and pointedly not pushing, because it promised Mensah it would not. And the only security out here is a SecUnit working far below capacity and refusing to say what happened to it.