My Enemy, My Ally — Diane Duane

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