Worf's ponytail makes its debut here. He would keep it for the remainder of this series and keep when he moved on to Star Trek: Deep Space Nine (1993). Michael Dorn wanted the change because he was tired of "looking like a Klingon who's just been to the beauty salon!"
Originally, Doctor Crusher was the kidnapped crew member, but the staff thought that Troi's empathic powers were more suited to an espionage plot.
When Ensign DeSeve talks to Picard, he erroneously calls him "Commander". This is because on Romulan ships, the highest ranking officer is called "Commander" rather than "Captain".
It is revealed that the Romulans use an artificial quantum singularity as a power source on their starships in the 24th century. This would explain why the warp core of the Romulan science vessel featured in The Next Phase (1992) imploded as opposed to exploded. This would also be a crucial plot point in Timescape (1993), later in the season.