In 2264, it was the attendant assigned to nurse the human Winston back to health following his near-deadly crash on Vendor. During their association, as was natural for Vendorians, it absorbed Winston's personality, mannerisms, beliefs and feelings, to such an extent that it was considered a deviant among its own people.
It was considered a "non-producer" in its society, which amplified feelings of being an outcast. After Winston died, it sought something of value to do. When Romulans sneaked across the Romulan Neutral Zone and broke the planet's quarantine seeking to recruit spies, it accepted their offer.
In 2269, it was assigned to lure the USS Enterprise across the Neutral Zone to a pair of awaiting Stormbird-class Romulan battlecruisers. It piloted Winston's damaged transport through areas of meteor storms until it was detected by the unsuspecting starship. "Winston" was beamed aboard, whereupon it provided the human's identity tape. Its voiceprint, fingerprints and retina patterns were identical to Winston's but medscanners detected minute differences in its biology which Leonard McCoy attributed to calibration errors.
It was not expecting to find Winston's fiancée Anne Nored serving aboard the Enterprise. It told her they could never be together, but its bond with the human Winston meant that it also loved her.
It assumed the form of Captain James T. Kirk and diverted the Enterprise into the Neutral Zone. It subsequently posed as McCoy and a biobed. It then became an engineering technician and sabotaged the ship's deflector shield generator so that the starship was defenseless against the Romulans. But when faced with the actual destruction of the ship, and the subsequent loss of life, including that of Nored, it couldn't go through with it — it became a functional shield generator to protect the ship. When the shield came up, Kirk rapidly fired on both Romulan ships, then immediately departed the Zone.
It surrendered to Kirk on the bridge. It agreed to stand trial for espionage, and was willing to discuss some sort of future with Nored. She requested to be assigned as its security detail. (TAS episode & Log Two novelization: The Survivor)