In 2367, he was involved in a complex plan to bring a Romulan spy in the Federation back to the Romulan Star Empire. The spy, Sub-Commander Selok, had assumed the identity of Vulcan Ambassador T'Pel and been collecting information on the Federation for years, using her high security code and access to secret data. To bring her back, Mendak arranged a meeting between him on board the IRW Devoras and T'Pel on board the USS Enterprise-D.
Mendak pretended to be willing to start negotiations between the Federation and the Romulans and invited T'Pel to his ship; in the process of her beaming aboard, he faked a transporter accident to make the Enterprise-D's crew consider her dead. The Starfleet crew eventually discovered the truth and confronted the Devoras in the Romulan Neutral Zone, but Mendak had prepared for this with four additional Warbirds ready for his support, forcing the Enterprise-D to retreat. (TNG: "Data's Day")