The Battle of Frontier Day was the final engagement between the United Federation of Planets and the remnants of the primary Borg Collective as well as the conclusion of the Changeling conspiracy.
Both seeking revenge upon the Federation, a rogue faction of Changelings had allied with the surviving Borg to destroy their enemies, culminating in the takeover of Starfleet by the Borg on Frontier Day. However, this proved to be the final downfall of the Borg when the rebuilt USS Enterprise-D was returned to service to end the Borg threat once and for all.
Prelude[]
In 2375, the Dominion War came to an end with the Battle of Cardassia and the Treaty of Bajor. (DS9: "What You Leave Behind") However, not all of the Changelings were pleased with the peace following the war and under the command of Vadic, who had suffered greatly as a part of Project Proteus, these Changelings broke away from the Great Link. (PIC: "Dominion", "Seventeen Seconds")
Following this divide in the Great Link, Worf was contacted by Odo, and warned of Vadic's break away, rogue terrorist faction. Out of the mounting fear of reigniting the Dominion War, Starfleet was not able to acknowledge their existence, and Vadic's forces slowly integrated themselves into the highest echelons of Starfleet itself, by using the very abilities they were given against their former tormentors. (PIC: "Seventeen Seconds", "Imposters")
In 2378, the Borg Collective was decimated to the point of near-extinction when an Admiral Kathryn Janeway from the future infected the Collective with a neurolytic pathogen, combined with the USS Voyager destroying the Borg transwarp network. (VOY: "Endgame"; PIC: "The Last Generation") The Borg Queen eventually made contact with Vadic and the two formed an alliance to destroy Starfleet and exact revenge by using the latent Borg DNA that had been placed in Admiral Jean-Luc Picard when he was transformed into Locutus of Borg and which had been passed down to his son Jack Crusher. Recovering Picard's original Human body from Daystrom Station, the Changelings used the fleet's transporter system to incorporate the Borg DNA into all Starfleet personnel, allowing for their assimilation via remote signal when the time came. (PIC: "Võx")
However, the plan experienced challenges when Picard and the crew of the USS Titan-A began working to protect Jack. In addition, they received help from Worf and Commanders Ro Laren and Raffaela Musiker who were investigating Changeling conspiracy. This culminated in the deaths of Vadic and her forces and the destruction of the Shrike, but in trying to understand what was going on with him, Jack was drawn to Jupiter where he met with the Borg Queen aboard her Borg cube where he allowed the Borg Queen to turn him into Võx and the command signal for the Collective. (PIC: "Imposters", "The Bounty", "Dominion", "Surrender", "Võx")
The battle[]
The Assimilation Begins[]
After discovering the truth, the USS Titan-A raced back to Earth, arriving as the Frontier Day celebrations, led by Fleet Admiral Elizabeth Shelby aboard the USS Enterprise-F were underway. As Shelby began to demonstrate the new fleet formation mode, the Borg command signal activated, beginning the assimilation of both the fleet and the Starfleet personnel aboard the ships. Only those under twenty-five years of age were immune to the Borg takeover and they quickly became targets of their assimilated crewmates with Admiral Shelby and Captain Liam Shaw being amongst those killed in the fighting.
The USS Excelsior's crew managed to regain control of the ship's bridge, but the automated system quickly retaliated by moving the ship into the firing line of the USS Cochrane, USS Hikaru Sulu, USS Intrepid, and USS Luna which destroyed the ship. As Commanders Seven of Nine and Raffaela Musiker stayed behind on the Titan to try to retake the ship -- Seven having been promoted to acting captain by the dying Shaw -- Admiral Jean-Luc Picard, Captains William Riker and Worf, Commodore Geordi La Forge, Doctor Beverly Crusher, Councilor Deanna Troi, and Data escaped in a maintenance shuttle, making their way to the Fleet Museum where they reactivated the rebuilt USS Enterprise-D which, due to not being connected to the Starfleet mainframe, was the only ship immune to the Borg takeover. (PIC: "Võx")
Defending Earth[]
As the Borg assimilated ships targeted Earth, it was defended by Sol Station and Earth's planetary shields which would not be able to hold up to that kind of firepower for long, Earth's orbital defense platforms having been destroyed. President of the United Federation of Planets Anton Chekov issued a planetary distress call warning all ships of the Borg threat and to stay away from Earth which was facing imminent destruction with no one able to figure out how to break the Borg control.
On the Titan, Seven managed to retake the bridge with the help of Musiker and several older crewmembers. In order to buy Earth more time, Seven had the cloaking device stolen from the HMS Bounty engaged as the automated system worked on line of sight, allowing the crew to regain control of their ship. Using every prefix code they had to scramble the shields of as many ships as possible, the Titan made attack runs on the Borg-controlled vessels. However, the Titan alone was only able to make a minimal difference. Eventually, the assimilated crewmembers broke out of where they had locked up and destroyed the cloaking device, rendering the Titan vulnerable to the Borg automation again, before heading to the bridge to confront Seven's crew.
Despite the valiant defense of both the Titan and Sol Station, Sol Station eventually fell, taking with it the planetary shields. The Borg ships then began targeting every major population center on the planet for destruction from orbit. (PIC: "Võx", "The Last Generation")
The End of the Borg[]
Deducing that the Borg had to be close for their command signal, the Enterprise located the Borg Queen's cube hidden in the center of Jupiter's Great Red Spot. The cube proved to only be 36 percent operational with what power it did have being used to broadcast the hive mind. After detecting the Enterprise, the Borg Queen disarmed her ship's weapons and lowered her shields in an invitation to Picard aboard to witness her victory. Picard decided to beam aboard the cube to find his son while Riker and Worf joined him in search of the beacon that was broadcasting Jack's voice as a command signal.
Picard, using his residual link to the Collective, was able to track Jack and the Borg Queen to the unimatrix array where Picard unsuccessfully attempted to reach his son, now Võx of Borg. At the same time, Riker and Worf found most of the Borg drones to be dead, having been consumed in by the Queen in her desperation to survive. The two eventually located the Borg broadcast emitter and sent its location to the Enterprise before having to fight off a few drones that were still alive.
The Enterprise discovered that the beacon was located at the very heart of the cube, a seemingly impossible location to fly to even for a computer or the best pilot. However, Data insisted upon his ability to fly and La Forge, in command of the ship while both Picard and Riker were on the cube, agreed. Although the cube turned its weapons on the Enterprise, Crusher managed to destroy the defense turrets. Data was able to fly the ship through a treacherous path to the middle of the cube where the crew found the beacon, but were faced with a dilemma as it was tied into the very substructure of the cube, meaning that destroying the beacon would destroy the cube itself, Picard, Riker, Worf and Jack still onboard.
With no other choice under the circumstances, the Enterprise fired on the beacon, triggering a cataclysmic chain reaction. In a final attempt to reach his son, Picard linked with the Borg hive mind and successfully convinced Jack to break free and return to his family. Although the Enterprise was initially unable to lock onto the four, Troi telepathically detected their position after hearing her husband's goodbye and flew the Enterprise in to beam them out. The Enterprise escaped as the Borg cube exploded, killing the Borg Queen and destroying what was left of the primary Borg Collective.
The destruction of the Borg cube and the rescue of Jack cut the Borg command signal, breaking the Borg's control over Starfleet just in time to save both the crew of the Titan and Earth. (PIC: "The Last Generation")
Aftermath[]
Following the battle, the full scope of the Changeling and Borg plot was fully exposed. The new head of Starfleet Medical, Admiral Beverly Crusher, was able to devise a fleet-wide transporter solution to remove the Borg DNA, bringing a final end to the threat of it being used again in the future. She was also able to devise a way to detect and expose the remaining Changeling infiltrators.
For their actions, the Enterprise command crew received pardons while Seven of Nine was promoted to the rank of Captain. In addition, the USS Titan-A was rechristened as the USS Enterprise-G. (PIC: "The Last Generation")