My Enemy, My Ally (Star Trek: The Original Series, No. 18)

Ael t'Rllaillieu is a noble -- and dangerous -- Romulan commander. But when the Romulans kidnap Vulcans, intending to harness their mind power through genetic experimentation, Ael decides on treason. Captain Kirk, her old enemy, joins her in a secret pact to destroy the research labaratory and free the captive Vulcans. When the Romulans discover their plan, the Neutral Zone seethes with schemes and counter-schemes, sabotage, and war!