Michael Jonas was a former member of the Maquis, serving under Chakotay's leadership, who subsequently joined the crew of the USS Voyager as an engineer when the Maquis and Starfleet crews were stranded in the Delta Quadrant.
In 2372, the death of fellow Maquis Kurt Bendera in battle with the Kazon caused him to question Captain Kathryn Janeway's policies regarding giving technology to other races in exchange for their assistance and protection. After witnessing his friend Hogan attempt to convince Voyager chief engineer B'Elanna Torres to contact Seska, who had defected to the Kazon-Nistrim, behind the captain's back, he secretly opened communications with Seska, intending to negotiate on his own terms. (VOY: "Alliances")
When Tom Paris successfully crossed the transwarp threshold, achieving warp ten, Jonas transmitted details of Voyager's new technology to a Kazon-Nistrim soldier named Rettik. (VOY: "Threshold") He also informed them when Voyager encountered the Cardassian Dreadnought. (VOY: "Dreadnought")
Fortunately, Voyager Chief of Security Tuvok detected Jonas's transmissions (which were sent via the power grid; the messages were encoded in the waste energy from the propulsion systems making them almost indistinguishable from galactic background noise), but was unable to locate the source. He reported this to Janeway and together they assembled a plan to expose the traitor. They secretly recruited Tom Paris to act as an agent provocateur, pretending to be dissatisfied with life aboard Voyager. Paris' act was convincing, and he even struck Chakotay while on the bridge.
In mid-2372, Seska ordered Jonas to sabotage Voyager's magnetic constrictors so that they would be forced to head to the Hemikek system and land on a planet there in order to seek materials to complete repairs. Kazon-Nistrim forces would then storm the ship on the ground. Around this time, Paris asked to be put off the ship and signed up with a Talaxian convoy as a pilot. Neelix became suspicious, not believing the reasons for Paris' departure. Janeway and Tuvok quickly acted to stop him from exposing their deception, and asked him to announce that they had found Paris to be the traitor. Jonas fell for the ruse and contacted Seska about these events. Kazon-Nistrim forces attacked the convoy and kidnapped Paris. Recognizing Paris' value, Seska tried to convince him to join them. Instead, Paris broke into their computers, learned the identity of the traitor, and then escaped. He was able to transmit the information to Voyager just as Jonas was preparing to sabotage the ship. Neelix continued his own investigation and also discovered Jonas' treachery. A struggle ensued when Neelix was trapped in main engineering with Jonas and attempted to prevent the sabotage, but Jonas was killed as he fell into a plasma fire near Voyager's warp core. (VOY: "Investigations")