The newly promoted Lt. Ro Laren (Michelle Forbes in her first appearance since the 5th season) returns to the Enterprise and immediately embarks on an undercover mission to infiltrate the Maquis - a group living in the Demilitarized Zone who is against the Cardassians since the peace treaty since they are now on part of Cardassian territory instead of Federation (they will become a formidable foe in DS9 and VOY).
Ro only takes this mission out of loyalty to Picard, being uneasy helping the Cardassians since she is Bajoran. After learning more about the Maquis, she grows more sympathetic with their cause. And due to this conflict of loyalties, Ro Laren is torn . . . but must soon make a choice.
Trivia note: Deep Space Nine is mentioned here.