Star Trek The Next Generation: The Defector

Answering a request for asylum from a Romulan scout ship, the crew beams aboard. Although he claims to be a low-ranking logistics officer, the vessel's pilot, Setal, shares a startling revelation: the Romulans have established a base on Nelvana III, from which they plan to launch a major offensive to reclaim the Neutral Zone. Informing Picard that the assault against the Federation will begin in 48 hours, the Romulan defector begs the captain to destroy the base to prevent a catastrophic war. 
Without firm evidence supporting Setal's claims, Picard fears that Setal's story may be a Romulan ploy to trick the U.S.S. Enterprise into initiating war. His suspicions are heightened when Setal informs Picard that he is really a Romulan Admiral named Jarok, who once led a massacre of Federation outposts. Although Jarok now claims that he only wants to keep the peace, Worf alerts Picard that Klingon ships in the area have warned him not to trust the Romulan, since his race had once tricked a great Klingon commander into appearing the aggressor in a war. 
With time running out on a volatile situation, Picard meets with his security officer, Worf, and then leads the U.S.S. Enterprise through the Neutral Zone to Nelvana III. When a closer probe of the planet reveals no Romulan power base, Jarok is dumbfounded. 
Suddenly, the U.S.S. Enterprise is surrounded by three Romulan warships and ordered to surrender by Tomalak, a commander who once confronted Picard over a skirmish in the Galorndon Core. Picard realizes that Jarok, who had recently been reassigned to a position of lower rank, was used by the Romulans to lure the U.S.S. Enterprise into a trap. The information about the base was all just a ruse to test Jarok's loyalty to his leaders. 
Picard refuses Tomalak's demands that the crew surrender and, at a prearranged moment, three Klingon warships uncloak, surrounding the Romulan ships. The odds stacked against him, Tomalak backs down and retreats, promising Picard that they will meet again. Knowing that his defection has cost him his family and his homeland, a despondent Jarok commits suicide.