The Enterprise docks at Deep Space Nine to assist with Bajoran equipment repairs. While there, a Yridian who traffics in information (James Cromwell, nearly unrecognizable) approaches Worf and says that his father Mogh did not die at Khitomer 25 years ago but instead was taken prisoner and is still alive in a Romulan POW camp. Meanwhile, DS9's Doctor Bashir (Siddig El Fadil) comes aboard the Enterprise to run an experiment and becomes fascinated with Data. (Bashir is mostly interested in Data's human personality traits as programmed by his creator Soong; this plays into the story's theme about fathers and sons.)
"Birthright, Part I" is TNG's only explicit DS9 crossover episode. Looking back ex post facto, it's interesting, almost funny, to revisit this version of Julian Bashir, so greenly wide-eyed and enthusiastic, knowing how much more serious and grown-up he will become. That's really neither here nor there as far as this episode is concerned, but it was something that caught my attention.
What's of more relevance is when Data gets zapped by an energy beam and is knocked unconscious for about 45 seconds, during which he has an intriguing vision that includes his father. He spends much of the rest of the episode trying to reconcile the meaning of the imagery. He creates dozens of paintings of what he saw in the vision and ultimately decides to recreate the circumstances of his unconsciousness. While Data's subsequent exploration of this dream realm gets a little heavy on arty, new-agey mumbo jumbo and imagery, there's a resonance in the message Soong has for him that feels like a rare moment of actual growth for the character. By the end, Data realizes that he should shut down every night and try dreaming, to see where it might take him.
Meanwhile, Worf struggles with the idea that his father might be a live prisoner rather than having died at Khitomer (which, of course, would be a grave dishonor; what isn't a dishonor for Klingons?). This is mostly setup for the second part, but what we have here is reasonable table setting, as Worf travels with the Yridian to the Romulan prison camp and discovers that although his father did in fact die at Khitomer, there's an entire colony of Klingons that survived. Before this final revelation, however (which kind of feels like a bait-and-switch), "Birthright" is about two orphaned sons who are confronted with new things about their fathers that could significantly alter their own self-identities.