He was a chief operative of the Nyrians, and was the first to arrive on Voyager. He pretended that he did not know how he got there. He said that he was abducted, but was in reality using a translocator to take the place of the crew and transport them to an environment biosphere on the Nyrian ship.
The plot was foiled when the crew was able to escape by using a translocator, and Dammar was transported to a ice age environment. The Nyrians surrendered and the crew retook Voyager. (VOY: "Displaced")