"Dark Frontier" is a feature length episode of Star Trek: Voyager, the 15th and 16th episodes of the fifth season. This episode originally aired as a feature-length episode that was later broken up into two parts for reruns in syndication.
After Voyager manages to destroy a Borg probe, Seven of Nine finds data nodes filled with tactical information among the debris. With it they locate a heavily damaged sphere nearby, and Janeway decides to plan a "heist" — invade the Borg vessel while its defenses are down and take its transwarp coil, which will shave about 20 years off Voyager's journey. The crew will create a diversion, then send an away team in to steal the technology. Hoping to find information that will give them a tactical edge, Janeway assigns Seven to study her parents' field notes that Voyager recovered from the USS Raven.
Once she begins studying her parents' logs, Seven remembers their encounters with the Borg. She was only a small girl at the time, but she vividly recalls their fascination with the mysterious Collective. Meanwhile, Voyager catches up with the sphere. The sphere's shields and transwarp drive will be off-line for the next 72 hours, allowing the crew only a short time to plan and execute the mission.
During a holographic simulation, Janeway and the others practice their mission down to the second. They have only two minutes to find and extract the transwarp coil after the sensor grid aboard the Borg sphere is disabled. Their simulated mission fails when Janeway and her team take too long and the Borg regenerate their sensor grid and detect the intrusion. After leaving the holodeck, Seven is unsettled by her close proximity to the Borg, even if it wasn't real. When Naomi Wildman begins asking her questions about the Collective, Seven hallucinates that the Borg have accessed her neural transceiver and know about Janeway's plan.
Further research of her parents' mission lead Seven to conclude her parents underestimated the Borg, which eventually led to their assimilation. It was during this research that Seven discovered the Hansens' description of a bio-damper in their notes, which they used to move around undetected in a Borg vessel while conducting their field research. The Voyager team replicates the technology for use in their raid on the Borg sphere. Asserting that she is willing to risk her own well-being for the sake of the crew, Seven persuades Janeway to assign her to the away team despite the Captain's reservations.
The mission goes as planned until Seven once again hears the voice of the Collective callig her back to the hive. In a sudden change of heart, she refuses to transport back to Voyager with the others, and Janeway is forced to leave her before she is assimilated herself. The sphere returns to Borg space with Seven on board, and the Borg Queen welcomes her back to the Collective.
The Borg Queen informs Seven that the Borg "allowed" Voyager to liberate her from the Collective, but she won't be turned back into a drone because they want to study her memories. With her individuality intact, the Borg can look through her eyes to help them assimilate humanity. Meanwhile, Janeway discovers that Borg signals were being sent to Seven in her cargo bay alcove. Determined to rescue Seven, Janeway leads an away team in the Delta Flyer to find the Borg sphere that took Seven away. They use the stolen coil to take the shuttle into transwarp space, and incorporate multi-adaptive shielding based on the Hansens' field notes from the Raven to go undetected by the Borg.
As Seven is given her first assignment to assist in the assimilation of a species, she secretly helps four of the individuals escape. The Borg Queen scolds her, saying that her human emotions of compassion and guilt are weaknesses that are causing her pain. However, when Seven pleads with her to let the getaway ship escape, the Queen grants her request.
After the away team follows the sphere into Borg space, Janeway prepares to send a message to Seven through her Borg interplexing beacon. The Queen gives Seven a new assignment—to assist in the programming of nanoprobes that will assimilate humans. The Borg plan is to detonate a biogenic charge in Earth's atmosphere, and Seven will be turned into a drone if she does not comply. Taunting her, the Queen reveals that one of the drones standing next to her is Seven's father. Suddenly, Janeway's signal comes through, and the Queen discovers it.
As the Borg adapt to the Delta Flyer's shielding, Janeway is forced to beam to the vessel and de-activate the shield matrix around the Queen's chamber. While Paris eludes the other ships, Janeway confronts the Queen and orders Seven to leave with her. A dispersal field is formed around the chamber to block the Delta Flyer's transporter beam, but Seven tells the Captain to target the power node above the chamber. This disrupts the Queen's command interface, and Janeway and Seven are beamed to the shuttle. They quickly enter a transwarp conduit, but not before a Borg vessel sneaks in behind them. On Voyager, Chakotay and Torres fire a full spread of photon torpedoes at the conduit threshold, collapsing it just as the shuttle bursts through. The Borg ship is destroyed, and Seven is home again. Voyager uses the coil and gets 15 years closer to home before it burns out and is rendered useless.