Newly-promoted Lieutenant Ro returns to the Enterprise after a stint in Advanced Tactical Training (a grueling course that she is passing nicely, and for which a recommendation from Picard was instrumental in her even being accepted). At the same time, the Enterprise is en route to a briefing on the situation in the demilitarized zone on the Federation/Cardassian border, but their trip is rudely interrupted by a distress call from a Cardassian ship under attack by the Maquis, a group of renegade Federation citizens on the Cardassian side of the Zone which opposes the Cardassian presence.

The Enterprise arrives in time to thwart the attack, but while the Cardassian wounded are being treated, Gul Evek waxes indignant about the Maquis's continued attacks, and strongly implies that Starfleet is covertly encouraging their work. Picard indignantly denies that such is the case, and says they are trying to stop the Maquis. Evek says that their efforts are apparently meeting with "limited success", but has the same phrase thrown back in his face when the topic turns to Cardassian citizens in the Zone attacking other ships. Both agree that the situation is worsening, and Evek says that the Cardassian military will have to step in before long if things don't improve.

After he leaves, the Enterprise continues to its rendezvous, and Picard has his spirits further dampened by the arrival of Admiral Nechayev. Nechayev is skeptical of Evek's claims of victimhood, but professes to being very concerned about the Maquis's actions. While sympathetic to their desire for self-defense, current intelligence suggests they're planning more aggressive acts, and she says it must be stopped before the entire Zone goes up in smoke. They need an undercover operative to find the Maquis before they can stop them, and she wants Ro.

Ro is surprised, but Picard tells her she shouldn't be: her recent training, her species, and her past history with Starfleet make her an excellent and credible choice. She says that she's familiar with the Maquis's aims, as one of her instructors during training went off to join the group; but despite her loathing for Cardassians, she agrees to go, but tells Picard why: "to validate your faith in me."

Some time later, she enters a bar, and shortly thereafter hides as Data and Worf enter, "searching" for her and saying she's killed a Cardassian soldier. When someone else tells Data that Ro was here, but left for parts unknown, the pair leaves, and Ro comes out of hiding. She thanks Santos for his help, but asks him why he covered for her. He tells her "there are worse things in life than killing a Cardassian soldier," and the two talk about how such attitudes are refreshingly rare. She tells him she'd like to meet people who think as he does - and is promptly taken down by a blast from his phaser.

When she wakes, she sees Santos and two other people, a woman and an elderly man. They ask her if she actually killed the Cardassian (which she says she did), and ask who she is. She tells them her name and some of her history, (including the fact that her father was tortured to death by Cardassians while she watched) but then insists that she's departed Starfleet "unofficially" because she didn't like their appeasing attitudes towards the Cardassians. She asks if they are Maquis. "What if we were?" "If you were, I would ask if I could join you, and help you fight."

Macias (the old man) sends the other two away to check out her story, and takes Ro for a walk, despite her shock at being given such a wide berth. He shows her the village where they are, and says that Starfleet doesn't understand the situation in the Zone. The Cardassians are determined to make the Federation colonists leave, any way they can - "but no one seems to SEE that, except the Maquis." He goes to a replicator and gets hasparat, a Bajoran dish that he's developed a taste for over the years despite being human. Ro says that her father used to make a very strong hasparat, and he asks her to make one for him. Meanwhile, the others return with news: they've checked Ro's story out, and it's true.

Later, Ro meets with a larger group, apparently the ruling committee of this Maquis cell. There are rumors circulating about the Cardassians; specifically, that they're shipping biogenic weapons into the Zone somehow. This is too big to be ignored, and they all agree that a preemptive strike is necessary. The lack of medical supplies is a major concern, but Ro says that she can get them - by stealing them from the Enterprise, since she knows their security codes. Everyone is incredulous, but Macias trusts her, and insists that they let her try. Kalita, the woman who interrogated her at the start, remains skeptical and insists on coming along. Ro, however, has no problem with this: "I could use an extra hand at the controls - and a witness."

Ro's first "miracle" in the Maquis ship comes at the border into Federation territory, when she crosses at a sensor buoy, using its security information to disable the alarms. She then sends a false distress signal to the Enterprise, claiming to be a disabled science vessel in the Topin system. The Enterprise heads towards that system, but Data cautions that sensors will be useless due to interference from a protostar in the region. They go in, using directional sensors only.

As Ro and Kalita wait, the Enterprise arrives, and begins a painfully slow search of the system. Another distress call suddenly arrives, and they move towards it - but this time, Ro uses the sensor beam to maneuver towards the Enterprise. Since they can't beam the supplies through the shields, Ro claims to be able to take the ship itself through the shields by exploiting a weak point in the system. In reality, she sends a piggybacked message to Picard with the second distress call, and although garbled, enough of it gets through that Picard decides to let the ship in. Ro takes the ship in, beams out the supplies, and gets out in a hurry, with Picard deciding to fire somewhat behind the ship to make it look good.

When they return, Kalita's attitude towards Ro has made a complete turnaround, and Macias is beaming. "We've learned to be cautious of strangers," he confides, "but I've known since I first met you - that in your heart, you're one of us."

Ro is now trusted enough to get her own ship and free access to it, and sometime later uses it to meet Picard on the Enterprise. Picard dismisses the rumors of the biogenic weaponry, but muses that it could be used to set up a decoy, a "target" so tempting that many Maquis cells would combine forces to defeat it, but instead decimating the Maquis forces at a stroke. He suggests that Ro provide her cell with fake intelligence "confirming" that parts for these weapons are being provided through third parties, thus setting the plan in motion. Ro is not sure, worrying about the idea of leading everyone into a trap; but when Picard presses her, she says that even if she has misgivings, "they will not prevent me from carrying out my mission." Picard is pleased: "I knew that I could count on you."

She brings that "intelligence" back to the Maquis, and they take the bait as planned, with Macias contacting other cell leaders. Later, after he has done so, he talks to Ro and tries to bring up her spirits by planning a celebration. When he mentions playing the bellaclavion, a Bajoran instrument, Ro suddenly gets very pensive. Her father used to play the clavion to keep monsters away, and always made her feel safe with its playing. "When he died ... I realized even he couldn't make all the monsters go away." Macias decides that there's no need to wait for a celebration - why not get the food for one right now, for no reason at all?

The two head for the market, but suddenly three Cardassians appear and open fire on the colony in an ambush. Ro holds her own, even killing one of them, but Macias is killed in the attack. Ro reaches him too late to save him, but in time to hear his last words: "When an old fighter like me dies, someone always steps forward, to take his place..."

Later, Ro meets with Picard in the same bar where she joined the Maquis, under the cover of Picard trying to buy her "services". She tells him that the mission must be canceled, that the Maquis haven't taken the bait - but when he presses her, her lies become less convincing, and he wants to know what's going on. Ro says she's no longer sure where she stands; she doesn't want to disappoint Picard, but also doesn't want to betray these people. Picard is adamant, however; at this point, everyone's already committed, and backing out now would ruin everything Ro has worked for. Picard is calm, but unrelenting: "I could put you before a Board of Inquiry for lying to me about this mission; I will certainly have you court-martialed if you sabotage it." With no choice, Ro agrees to do her part, but Picard insists on sending Riker along (posing as a relative) as extra insurance.

Later, the Maquis attack is imminent, and the Enterprise is waiting with a strike force in a nearby nebula for the Maquis to cross the border. Ro and Riker are in one ship of the Maquis force, about to cross the border themselves - but then suddenly, she stops the ship, holds Riker at bay with a phaser, and fires a particle beam into the nebula, exposing the Federation ships. As the Maquis abort the attack without crossing the border, Ro lets her ship cross over and head for the Enterprise.

As Picard and Worf prepare to take her into custody, Ro tells Kalita on another ship to prepare to beam her aboard. She tells Riker that she's joining the Maquis; "It's been a long time since I really felt like I belonged somewhere." She asks Riker to do her one favor: tell Picard that she's sorry for letting him down. With that, the two bid farewell, as Ro is beamed away.

When Riker returns to the Enterprise, he tells Picard of what happened, and hands him the report - but Picard won't take it. Riker leaves Picard's ready room, and Picard sits, devastated and shocked by Ro's final betrayal.