My Enemy, My Ally

Ael, a Romulan commander who once spared Kirk's life rather than start a war, comes to the Federation for help: a secret facility is breeding Romulan mental powers into captive Vulcans, and she needs the Enterprise to destroy it from inside Romulan space. What begins as a covert alliance turns to open combat when Ael's own son tries to betray her, and by the end she is a traitor to the empire she was trying to save.