In 2367, the Romulan Empire requested T'Pel as an intermediary in peace negotiations between the Romulans and the Federation. She was transferred from USS Zhukov to the USS Enterprise-D on their way to the Romulan Neutral Zone. During her stay on the Enterprise she summoned Lieutenant Commander Data and attempted to make use of his Priority 3 clearance on the ship to gain information about the deflector shields' field strength at maximum output, using her access code of Kappa Alpha 4601704. Data responded that he should report any inquiry regarding restricted information to the captain, and T'Pel replied that she was simply testing if his security safeguards were adequate.
The ambassador apparently died during transport from the Enterprise to the warbird IRW Devoras. Data, however, found out that the transporter accident was staged and surmised that T'Pel was actually alive and being held hostage by the Romulans.
It was later discovered that T'Pel was in fact Subcommander Selok, a Romulan agent assigned to infiltrate the Federation diplomatic corps - the peace overtures and transporter malfunction carefully planned in order to facilitate her extraction. (TNG: "Data's Day")
Background information Edit
Ambassador T'Pel was played by actress Sierra Pecheur.
The script for "Data's Day" describes the pronunciation of her name as "TA-PELL". Of her first appearance, it says, "She is Vulcan, middle-aged, and has the cool, regal bearing common to her species." 
Her headwear was created by "Harry Hatz".
In the Pocket TNG novel The Romulan Prize, Selok manages to acquire a special computer access isolinear chip that allows an individual to bypass the security lockouts on Starfleet computer databases during her time in Federation space. This chip is given to Commander Valak in 2368, allowing him to access the Enterprise's computer systems after capturing the ship. He looks for information on Hermeticus II, which is located somewhere in the Romulan Neutral Zone and is classified as a quarantined world in Starfleet records. This information makes the Romulans suspicious, believing that a military base is present in the region.