At some point during 2371, Soong discovered evidence of the Borg, a cybernetic species whose advanced technology he considered a possible vindication of his theories about artificial life. This evidence, along with his disdain for the Alliance, convinced him to seek the assistance of the Terran Rebellion. The Rebellion, convinced by Soong that the Borg were likely benevolent and might be willing to assist them in defeating the Alliance, smuggled him out of Alliance custody.
In September 2371, Soong accompanied Vash to Celtris III to seek the services of Jean-Luc Picard, a Terran archaeologist for hire in the employ of Celtris III's planetary governor Gul Madred, in finding and establishing contact with the Borg. Picard was initially skeptical and refused to sign on to the endeavor, only to change his mind after the brutal beating of his friend Wesley by two Klingon security guards and the ransacking of his ship, the Stargazer. The three parted ways, with Vash staying behind to conduct a mission for the Rebellion while Picard and Soong ventured into the Romulan Neutral Zone in search of the Borg.
The pair began searching the Romulan side of the Neutral Zone, discovering a series planets with craters where listening stations had previously been, indications of the Borg's powerful technology. Over a period of weeks, the two were able to determine the course of the Borg vessel and track it to the orbit of Carraya IV. At Soong's insistence, the two beamed over to the Borg vessel, only to be soundly ignored by the drones aboard. When Soong disturbed one of the regenerating drones, he and Picard were targeted for assimilation by a trio of others, including an assimilated Vash. While Picard insisted upon escape, Soong refused to leave, feeling that he would lose his chance to open a dialogue with the Borg. Sometime after Picard left, Soong was assimilated into the Borg Collective, designated as Locutus of Borg, and assigned to be the "speaker" for Humanity. (TNG - Mirror Universe novella: The Worst of Both Worlds)