Captain Picard is assimilated to the Borg in Star Trek Next Generation

In Star Trek: The Next Generation, Captain Jean-Luc Picard was assimilated by the Borg, a powerful cyborg race that sought to assimilate all other life forms into their collective. This event occurred in the episode "The Best of Both Worlds" which is a two-parter from the show's third season.

In the episode, the Borg launch a massive attack on the Federation, destroying several colonies and starships, and Captain Picard and the USS Enterprise are sent to stop them. The Borg abduct Captain Picard and surgically assimilate him into the collective, giving him cybernetic implants and turning him into a Borg drone known as Locutus. With the help of the crew, Riker manages to free the Captain and restore him back to normal but the process causes him great distress as he was forced to witness and participate in the assimilation of several planets. The experience left a profound impact on Picard and he was haunted by it for a long time. This event is considered one of the most iconic moments in the series, as it was a turning point for the character of Jean-Luc Picard and the show itself, highlighting the dangers and the stakes of their missions and showing the darker side of the Federation. The impact of Captain Picard's assimilation was felt throughout the entire series, and it was revisited in later episodes and films of the franchise.

In the episodes following "The Best of Both Worlds", Captain Picard struggles with the trauma of his assimilation, as well as the guilt of the things he did and the things he was forced to witness as Locutus. He receives counseling from his crew, but most notably from Deanna Troi, the ship's counselor.

The Borg, who had previously been portrayed as a faceless, unstoppable enemy, became a more personal threat for Captain Picard after his assimilation. He continues to face the Borg in later episodes of the series, and he even has to confront Locutus, his Borg persona, in an episode called "The Inner Light", where he lives an entire lifetime on a distant planet and also where he is able to confront some of the things he did as Locutus.

It also provided a narrative arc for the series by making the Borg a more personal enemy, and giving them a more prominent role throughout the show. The crew of the USS Enterprise, have to face the Borg again in the series and also in the films and they always have to fight the impulse of assimilating the crew. It's also often referred as a turning point in the franchise and the series, as it raised the stakes of the show and highlighted the dangers of deep space exploration and the cost of defending the Federation.