DeSeve defected due to a desire to live in a society that had a "strict moral compass", believing the Romulans to possess this quality. After trading information on Federation starship engineering with the Romulan forces, DeSeve chose to enlist in the Romulan military, training as an engineer.
Twenty years later, DeSeve had risen to the position of subcenturion in Engineering, but felt stifled; with no further chance of career advancement in the Romulan Empire due to his status as a defector, and feeling lonely and untrusted, DeSeve fell in with the Romulan underground. Upon deciding to return to the Federation, he was given a message from Ambassador Spock to deliver to Federation authorities.
DeSeve was picked up by the USS Enterprise-D in 2369. He was immediately arrested, and charged with treason. DeSeve fully accepted that a court-martial and dishonorable discharge were the most likely outcomes of his return to the Federation, followed by a term of detention in the New Zealand Penal Settlement.
He presented Spock's message to Captain Jean-Luc Picard, who acted upon it with some reluctance. The Enterprise attempted to rendezvous with a Corvallen freighter, but found that it had been destroyed; locating its debris resulted in DeSeve correctly identifying a magnetic distortion sensor reading as a Romulan Warbird. The Warbird, temporarily under the command of Starfleet Commander Deanna Troi (posing as Major Rakal of the Tal Shiar), beamed its cargo - and Troi herself - to the Enterprise. This cargo was soon revealed to be Romulan Vice-proconsul M'ret and his aides N'veran and Revaik.
Shortly after retrieving the Vice-proconsul, the Enterprise was critically damaged by a Romulan radiation grenade; in addition, the transporter beam from the Romulan Warbird had damaged the ship's power couplings, and the warp core was threatening to breach. DeSeve offered to defuse the grenade, knowing that his knowledge of Romulan technology made him the only person able to do so. Crawling through a Jefferies tube, he was able to deactivate the grenade and force the power couplings closed - however, he suffered extreme radiation poisoning from the device, and succumbed to his injuries moments after saving the ship.
Captain Picard later recommended to Starfleet Command that all charges against DeSeve be dropped, and ordered that he be buried in space, with full military honours - although he had not specified which military honours would be used, he suspected that it would not matter. (TNG episode: "Face of the Enemy", TNG - The Sky's the Limit short story: "Turncoats", ST website: Startrek.com)