In this episode, the officers of the FederationStarfleetstarshipUSS Enterprise-D encounter Ishara Yar (played by Beth Toussaint), the sister of their former shipmate Tasha Yar. While joyous at the opportunity to interact with someone who helps them reconnect with the memories of their former comrade, they soon discover that Ishara has a different agenda.
The Enterprise responds to a distress call from the Federation freighter Arcos which has suffered from engine failure and taken emergency orbit around the colonized planet Turkana IV, the birthplace of the Enterprise's former chief of security, Tasha Yar. In orbit, the Enterprise crew find the Arcos suffered a warp-core breach and was destroyed, but an escape pod appears to have been launched and start searching the planet for it. They learn that the planet's colonists turned against each other, destroying their surface dwellings and forcing the survivors underground. A warning from the Potemkin from six years earlier warns that the remaining colonists execute any trespassers on the planet.
The crew makes contact with one of two warring factions, the Coalition. Their leader Hayne reveals that the other faction, the Alliance, hold the Arcos survivors hostage, and offers the Enterprise their support in exchange for Federation weapons, which Captain Picard rejects. Hayne, on learning that it is the Enterprise in orbit, instead offers the services of Ishara Yar, Tasha's sister, knowing of Tasha's past service aboard the ship. Intrigued, Picard accepts Ishara aboard. Ishara, though disrespectful of her sister abandoning the Coalition's cause, offers to prove her identity to genetic testing and offers knowledge she has of the Alliance tunnels to gain the crew's trust. Through this, Ishara is readily accepted as part of the crew, and Data discusses his past memories he has for Tasha with Ishara.
When the Alliance announces they will execute the Arcos crew in 20 hours, the Enterprise hurries on its plans to infiltrate the Alliance territory. Dr. Crusher and Data remove a proximity sensor that Ishara, like all members of the Coalition, was implanted with that would activate if she had approached the Alliance security sensors; Ishara gives Data the sensor as a memento. The away team transports to the planet and executes their plan, safely rescuing the hostages back to the Enterprise, but Ishara is lost in the confusion. Data searches for her and finds her attempting to disable the Alliance security grid; Ishara reveals that once the grid is down, a large force of Coalition members are ready to attack the Alliance. Ishara's entrusting of the Enterprise crew was all a ploy by Hayne to allow her free access within the Alliance territory. After Ishara attempts to fire on Data, Riker distracts her, allowing Data to stun her and proceed to reverse her modifications on the Alliance systems.
Back aboard the Enterprise, Picard informs Hayne that as Ishara fired upon a Starfleet officer, they could hold her as a prisoner, but instead returns her to the planet. As Data escorts her to the transporter room, Ishara lets him know he was the closest thing to a friend that she had. Data considers his past relationships with both Ishara and Tasha as the Enterprise leaves orbit.