Seven of Nine dreams, her first according to the Doctor, of a beautiful forest. However, it turns out that this is a real subconscious realm inhabited by the minds of certain Borg drones during regeneration periods. Few drones possess the recessive gene required to experience the realm called Unimatrix Zero.
In the surreal, utopian Unimatrix Zero, Borg of various species and ages (including children) from throughout the massive geography of the Collective exist as their individual, unassimilated selves and interact with one another as such. While out of regeneration, they revert to normal drones and have no memory of their time spent together there. The Borg Queen knows about Unimatrix Zero, considering it a disease, and destroys drones discovered capable of visiting it. However, the process of detecting affected drones is very time consuming, and she is eager to find a faster method of finding and deactivating them. Those who are detected are deactivated, dismembered, and studied in an effort to speed up the search.
Seven of Nine journeys with Captain Janeway to the subconscious realm and reverts to acting more like an individual. It turns out that she used to visit Unimatrix Zero when she was still part of the collective. Her forgotten lover, Axum, has made contact with her so that she may help them; the inhabitants of Unimatrix Zero have developed a masking nanovirus which would inoculate them against being detected by the queen, but it can only be administered from the corporeal world. Seven soon becomes overwhelmed and discouraged by her newfound emotions and denies them at first, although she eventually comes to terms with them. If Unimatrix Zero cannot be stopped in the real world, the Queen plans to destroy it from within. Assimilated drones attack the subconscious "unassimilated" drones constantly.