Some of the characters from Ancestral Night make cameo appearances, but Machine isn't really a sequel. The story follows Dr. Jens, who enjoys her job on an interstellar ambulance crew dealing with a wide variety of medical emergencies. Some of these involve humans but there are so many species here that every call for help is a new challenge. The AI that runs the ship decides to take one last call before heading to the central system, the equivalent of jury duty in the US, and they find two ships locked together with the biological beings unconscious and the ship AIs confused and acting strangely. The take all of them back to Central Hospital and soon all sorts of things start happening in the usually boring station. Someone actually attacks them, the AIs forget things, and there are power failures for no reason, which could be deadly for the patients. But what Dr. Jens finds strangest of all is that there is a ward restricted to only a few personnel and no one knows who the patients are or what treatment they are receiving. Somehow it connects back to those two ships and the crippled AI, so Dr. Jens is determined to find answers.

While we met a few alien races and an AI in Ancestral Night, Machine introduces all sorts of fascinating aliens and AI minds. While on the station AIs are free to move anywhere through the system and mingle with others although only employees have access to patient rooms etc. There are even AI doctors that simply present their avatar in a room while consulting with patients or colleagues, some of which are working with the injured AI who's memory files were somehow shredded, probably to destroy any record of what happened with the two ships. The station is enormous and contains levels suited to alien atmospheres which you might expect. But would you expect the overall Administrator to be a tree? Yes it's a large tree which lives under a transparent dome so the local sunlight shines on it and it's roots reach all through the station's infrastructure and interface with the electronics. Unfortunately, the strange disasters afflicting the hospital have injured the Administrator and if Dr. Jens can't find the source it will die and which may destroy the entire station.

The story line is very different from Ancestral Night, but it's a great extension to the world building of the White Space universe. There are elements of hospital drama, sci-fi exploration of alien biology and culture, unpredictable acts of terrorism, and a backdrop of good old fashioned mystery. I personally thought the intrigue developed faster than in Ancestral Night and had fewer slow sections. Other reviewers thought Bear spent too much time on Jens' interior voice and that it wasn't important to the story. I also appreciate that with this series Bear is describing a diverse cast of aliens and that only a few are humanoid in appearance or thought processes which seems more realistic to the scientist in me. It's certainly true that human nature tends to discount the intelligence of those who look different from ourselves whether it's within our own species or not. Of note for audio "readers", Machine has a different narrator, Adjoa Andoh, who pronounces names differently than Nneka Okoye did which was jarring if listening to the two books back to back. Andoh has a long list of credits including Ann Lecke's Raven Tower that I reviewed a while back.