A supposed Vulcan ambassador to secret talks with the Romulans who turned out to be a Romulan agent, Subcommander Selok.
T'Pel presents herself as a Vulcan ambassador traveling aboard the Enterprise to a secret meeting with the Romulans inside the Neutral Zone. During the journey she asks Data unusually pointed questions about the ship's defenses, claiming she was only testing his safeguards.
In Data's Day, T'Pel appears to die in a transporter accident during the rendezvous with a Romulan warbird. Data's analysis shows the accident was faked and that she was beamed away by the Romulans. Picard learns that T'Pel is really Subcommander Selok, a Romulan intelligence officer who spent years working undercover in Federation society.