The Borg make frequent appearances in Star Trek: Voyager, which takes place in the Delta Quadrant, the region of space from which the Borg are believed to have originated. The Borg are first discovered by Voyager in the third season episode "Unity", Chakotay discovers a population of ex-Borg of various species. In "Scorpion", the Borg are engaged in a futile war against the much more powerful Species 8472. In one of the few instances of the Borg negotiating, in exchange for safe passage through Borg space, the Voyager crew devises a way to destroy the otherwise immune Species 8472. A Borg drone, Seven of Nine, is dispatched to Voyager to facilitate this arrangement as Captain Janeway refuses to allow the Borg to use neural implants for communication with the Voyager crew.
After successfully driving Species 8472 back into their fluidic space, Seven of Nine attempts to assimilate Voyager but is severed from the hive mind and stripped of most of her Borg implants, becoming a member of Voyager's crew. Seven of Nine's rediscovery of her humanity becomes a recurring theme throughout the series. Flashbacks and allusions in several episodes, such as "The Raven", establish that prior to her assimilation, Seven of Nine was Annika Hansen, the child of scientists who studied the Borg in the Delta Quadrant independent of the Federation.
In "Drone", an advanced Borg drone is created when Seven of Nine's nanoprobes are fused with the Doctor's mobile emitter in a transporter accident. The drone, who adopts the moniker "One", involuntarily sends a signal to the collective, bringing a sphere to Voyager. One destroys the Borg ship and lets himself die to protect Voyager from further Borg pursuits.
In "Infinite Regress" it was revealed the Borg have a device called a vinculum which connects all the minds of the drones, eliminates individual thoughts, and disseminates information to the Borg Collective.
In "Dark Frontier", Captain Kathryn Janeway decides to attack the Borg in the hopes of stealing a transwarp coil to aid Voyager's journey home. The Borg Queen learns of the plot and offers Seven of Nine a deal to spare Voyager in exchange for her rejoining the collective. Voyager recovers the transwarp coil and uses it, with the Delta Flyer, to save Seven from the Queen. Voyager uses the transwarp coil to travel 20,000 light-years before it burns out.
In "Collective", the crew of Voyager stumble upon a severely damaged cube that is holding Tom Paris, Neelix, Harry and Chakotay hostage. The crew later finds out that the ship is run by five Borg children who left their maturation chambers early. There are no living adult drones on board the ship due to a pathogen that somehow infected the entire vessel. The children sent a message to the collective that they were attacked. The collective replied and told them they were irrelevant. Seven of Nine is sent to assist in repairs and negotiate a trade for the prisoners. However, Voyager is soon attacked by the cube when the Borg children attempt to rip the main deflector off of the hull so they can rejoin the collective. When the cube's leader dies, Seven takes the children onto Voyager, has their non-essential Borg components removed prior to most returning to their home planets, some however choose to stay and become part of the crew.
In "Q2", Q's son places Voyager in danger and brings Borg onto the ship. Q intercedes, saves Voyager and angrily warns his son about provoking the Borg.
In the Voyager finale, "Endgame", an elderly Admiral Janeway from the future travels back in time, to aid in Voyager's return to the Alpha Quadrant through the use of a Borg transwarp hub. The hub is one of six such structures scattered across the galaxy, which allow Borg ships to traverse galactic distances in minutes. The future Janeway allows herself to be assimilated, delivering a neurolytic pathogen that disrupts the Queen's link to the collective. Janeway thus kills the Queen and helps buy time for the young Janeway to destroy the Borg Unicomplex: Voyager uses the transwarp hub to travel back to the Alpha Quadrant, destroying it as they do. On the way, a Borg sphere pursues and captures Voyager, but upon arrival at Earth, is destroyed from the inside by Voyager's transphasic torpedo.
Read more about this topic: Borg (Star Trek), Character History