https://en.wikipedia.org/wiki/Unification_(Star_Trek:_The_Next_Generation)

"Unification" is a two-part episode of the syndicated American science fiction television series Star Trek: The Next Generation, from the fifth season, which features Leonard Nimoy as Spock. It earned a 15.4 household Nielsen rating,[1] drawing over 25 million viewers,[2] making it one of the most watched episodes in all seven seasons of The Next Generation's run.

Hearing that legendary Starfleet officer Spock may have defected to the Romulan Empire, Picard travels to Vulcan to talk to Spock's father, former ambassador Sarek, who is near death from the ravages of Bendii Syndrome. In a rare lucid moment, Sarek discloses that Spock has long harbored hopes of peacefully reuniting the Vulcan and Romulan peoples, who once were part of the same civilization. Rather than committing treason, Spock actually may be initiating steps to achieve that peaceful goal. Determined to find the truth, Picard and Data, disguised as Romulans, set out for the Romulan homeworld. Upon finding Spock, Picard learns that the Vulcan is indeed on an unauthorized mission to reunify his people with the Romulans. Spock counts among his allies a Romulan senator named Pardek and the Romulan proconsul Neral.

Part 1[edit]
Starfleet Admiral Brackett informs Captain Picard (Patrick Stewart) that Ambassador Spock (Leonard Nimoy) has gone missing and an intelligence scan has placed him on Romulus, raising fears that he may have defected. Picard orders the Enterprise to Vulcan to speak to Spock's ailing father, Sarek (Mark Lenard), with whom Picard shares a close bond. Sarek gives Picard the name of Pardek, a Romulan Senator that he knew Spock had been maintaining a dialogue with for several decades. Lieutenant Commander Data (Brent Spiner) discovers a visual record of Pardek from a trade conference and confirms him as being the figure seen on the intelligence scan of Spock on Romulus.

On Romulus, Picard and Data locate the spot where the record of Pardek and Spock was taken, which Data determines is an intelligence bureau building. They wait until Pardek arrives, but when they approach him, they find themselves met by soldiers and taken to an underground cavern. Pardek arrives, explaining that Romulan security knew they were on-planet. Picard states that he is looking for Ambassador Spock, who emerges from a nearby tunnel.

Part 2[edit]
Spock demands that Picard leave Romulus, while Picard informs him of the Federation's concern over his "cowboy diplomacy" that he has engaged in, as well as the death of Sarek. Spock takes the news of his father's death stoically. Spock explains that during the peace negotiations with the Klingons decades earlier (Star Trek VI: The Undiscovered Country), he felt responsible for putting Captain Kirk and his crew at risk. Working alone to avoid similar issues, Spock is now engaged in a "personal mission of peace": the reunification of the Vulcan and Romulan people. He is working with an underground movement to achieve that aim. Pardek has asked Spock to come to Romulus to meet with the new Proconsul of the Romulan Senate, a young idealist who has promised reforms. Picard expresses concern that the willingness of the Romulans may be part of a larger ploy; Spock agrees but points out that if a larger plot is at work, it is best they play out their roles within it to uncover it.

Meanwhile, the Enterprise has arrived at Galorndon Core, discovering the three Vulcan ships. As the ships start to move towards Vulcan, the Enterprise blocks their path when they suddenly receive a medical distress signal - a distraction created by Sela. As Riker orders the ship to its source, they receive a broadcast from Romulus, where Spock reveals the true identity of the three Vulcan ships. A Romulan Warbird uncloaks, destroys the ships, and recloaks. Riker suspects that the Romulans would rather kill their own than have them captured by the Federation.

On Romulus, Data and Picard bid farewell to Spock. The Ambassador is intent upon his goal, realizing that it cannot be achieved through diplomacy or politics. The Captain offers Spock a chance to touch what Sarek shared with him, and the two mind-meld.