Picard: 15 ESSENTIAL Star Trek Episodes To Watch First (And Why)
15. “Balance Of Terror" (The Original Series - Season 1, Episode 14)
If we're going to be completists about this, you need to go back to the Original Series for the first episode featuring the Romulans. Yes it's a little bit of a stretch to say it's directly relevant to Picard, but it's the first major opportunity to see who the Romulans are, what their culture is and fundamentally, what their dynamic with the Federation and Starfleet is like.
In other words, it's an important step on the Romulan journey and the hints of their importance in Picard suggest knowing as much about them as possible will be key.
More specifically, the episode also includes a mysterious Romulan attack on a Federation outpost, a Starfleet captain taking it upon himself to make his ship and crew expendable to prevent a war and a heart-breaking sting in the tail. Which are probably all going to be a big part of Picard too.