The Romulan underground movement was formed sometime prior to the late 24th century on the planet Romulus by a group of Romulan citizens who opposed the Romulan Senate and who supported a Romulan-Vulcan reunification. Its methods and principles were similar to those of the Cardassian dissident movement which emerged in the Cardassian Union around the same time.
In early-2368, the group was greatly supported when Vulcan Ambassador Spock traveled to Romulus to join in the re-unification efforts. Spock's personal involvement was criticized initially by Jean-Luc Picard as "cowboy diplomacy" due to the risk and unofficial nature of his efforts. In that same year, the movement became known to the Romulan government and military. Senator Pardek was apparently a member of the movement, but was later revealed to be in league with the Romulan military, which was working to dissolve the movement. After Spock, Picard and Commander Data thwarted the Romulans' attempted invasion of Vulcan, they helped the movement members escape. The movement then promised to carry on their cause and teach others until the day free speech is permitted on Romulus. (TNG: "Unification I", "Unification II")
Later, in 2369, the movement continued to work toward re-unification. To that end, they abducted Lieutenant Commander Deanna Troi and used her to transport a defecting Romulan vice-proconsul and two aides to the USS Enterprise-D. (TNG: "Face of the Enemy")