"I am the beginning. The end. The one who is many... I am the Borg." - Borg Queen, Star Trek: First Contact
The Borg Queen is the central nexus of the Borg Collective in the Star Trek franchise. While the Borg race works as a unified mind, it is the responsibility of the Queen to coordinate and organise the actions and thoughts of the countless assimilated beings within the collective consciousness, much like how the queen of an insect hive manages the activities of all the hive's inhabitants. Unlike Borg drones, however, the Queen has been shown to display an individual consciousness and seems to possess a calculating, seductive and even sadistic personality when interacting with unassimilated individuals. As her title would suggest as well as her function, the Queen is the closest thing the Borg have to a leader, though if asked, she would state that she is the entire collective. The desires of the Queen reflect the drive and objectives of the Borg as a whole, their ultimate goal being to achieve a state of perfection by assimilating other races and their technologies to enhance the collective further.
As the Borg Queen organises the multitude of minds within the cacophony of the collective, the Borg will fall into chaos should the Queen perish and risk suffering tremendous damage or even total destruction. The Queen has in fact been killed multiple times, replaced by another Queen quickly after the former's death (though this process has never been shown in any Star Trek TV show or film).
Star Trek: First Contact
The Borg Queen is first revealed in the eighth Star Trek film and is portrayed by Alice Krige. The Queen personally oversees the Borg's second invasion of the United Federation of Planets and leads them in their attempt to alter Earth's past by stopping humanity's First Contact with the Vulcans. The Enterprise-E pursues the Borg sphere through the temporal vortex and destroys the sphere before it can destroy Zefram Cochrane's Phoenix Project. However, the Enterprise's shields and sensors are downed after travelling through the time vortex and the Borg Queen and her personal contingent of drones transport aboard the Enterprise without being detected. The Borg quickly take over the ship's Engineering section and almost gain control of the ship from there, but Data locks them out of the main computer using a fractal encryption code. The Borg then begin assimilating the Enterprise deck by deck and even manage to capture Data, whom the Queen speaks to directly. Standard assimilation methods will not work on the android but the Queen has devised a different approach: reactivating Data's emotion chip and grafting organic skin onto his body. Throughout the ordeal, Data tries to resist the Queen's powers of seduction but appears to fall under her sway.
In the end, Captain Picard confronts the Borg Queen directly and offers himself to the Queen in exchange for Data. Data feigns loyalty to the Queen and decrypts the Enterprise's computer, then prepares to fire on the Phoenix warp-ship as it begins its flight. Data intentionally misses the Phoenix and then smashes one of the warp core's coolant tanks. As corrosive coolant gas fills the room, Captain Picard tries to climb atop an assimilation frame and up a cable hanging from the ceiling. As the Borg Queen tries to pull Picard down, Data emerges from the coolant fog, his organic skin grafts dissolved away. He grabs the Queen and pulls her down into the coolant fog, which melts away her flesh. As the Queen dies, all the Borg on the ship suffer a tremendous neural feedback and all die. With the Borg dead, Picard reaches a control panel and vents the plasma coolant from the Engineering section. He spots the Borg Queen's body with its detachable head and spine now devoid of flesh, pure metallic skeleton lying loose from the cradle of the Queen's robotic body. The head still twitches with life until Picard picks it up, snapping the Queen's spine and ending her seemingly forever.
Star Trek: Voyager
Another Borg Queen appears in three two-part episodes of Star Trek: Voyager. In the fifth season story Dark Frontier and sixth/seventh season two-parter Unimatrix Zero, the Queen is portrayed by Susanna Thompson. In the series finale Endgame, the Queen is once again portrayed by Alice Krige.
The USS Voyager attempts to traverse a Borg-held nebula using technology provided by a future incarnation of Kathryn Janeway. Inside the nebula is a transwarp hub, a gigantic superstructure that maintains a network of transwarp conduits that the Borg use for rapid travel to various locations in the galaxy. One of the hub's conduits leads to Earth and Admiral Janeway urges her younger self to take the conduit and get home earlier than her history has recorded. Captain Janeway decides she would rather destroy the hub and deal a crippling blow to the Borg, but she and her older counterpart devise a plan to destroy the hub and still get Voyager home at the same time. For the plan to work, however, the admiral must sacrifice herself to the Borg Queen.
Admiral Janeway takes her shuttle into the transwarp hub and emerges at Unimatrix 1, the Borg's home base. Using a neural link, she makes contact with the Borg Queen to make a deal with her: she offers the information on her transphasic weaponry and in exchange the Borg escort Voyager back to Earth. The Borg Queen refuses to make a deal and her drones find the admiral's shuttle hiding within the Unicomplex. Admiral Janeway is beamed into the Queen's chamber and the Queen assimilates her, not aware that the admiral had injected herself with a neurolytic pathogen prior to arriving in Borg space. The pathogen spreads throughout the collective like wildfire, destabilizing the Unicomplex and causing the Queen herself to malfunction and deteriorate, her limbs falling off her body. The Queen collapses to the floor as the pathogen takes its toll on her and her head even slips loose of her body's cradle. Just as she loses consciousness, the Unicomplex explodes, destroying her, the admiral and all the Borg on board. The effect of the pathogen seems to spread throughout the collective, though the full extent of the damage is unknown. It is possible that the Borg were completely wiped out by the pathogen.