While a group of four stray cats trek through the ruins of an abandoned facility, one becomes separated from the others after falling into a chasm leading to an unpopulated underground city. The cat finds a lab where it helps download an artificial intelligence into the body of a small drone, which calls itself B-12. It explains it previously helped a scientist but much of its memory was corrupted and needs time to recover. B-12 promises to help the cat return to the surface and accompanies it further into the city. As they travel farther, the pair discover that, while the city is completely devoid of human life, their robotic servants, Companions, remain. With humans absent, the Companions have grown self-aware and have built their own society among the ruins of the city, but they likewise are trapped underground. The ruins are infested with Zurks, mutant bacteria that have evolved to devour both organic life and robots.