Walker's first appearance in the single-player storyline is when he meets with Jun'ko Zane and Edison Trent on the planet California Minor in the California system of Liberty space. Later, in orbit, he is set to accompany Trent and Juni to investigate the recent sightings of Rheinland fighters in the system when a distress call comes in from Willard Research Station. The station and the Liberty Dreadnought LNS Harmony had come under attack from numerous Rheinland fighters, gunships and bombers. With the help of Trent , Walker and Juni , the fighters are all destroyed. Immediately following the battle, Walker reports that he and his ship have been recalled to drydock, which he finds strange because his ships put into port recently and are not in need of repair or overhaul.
In the next mission, Juni contacts Trent and informs him that people around her are disappearing. Walker is missing, and his ship is recorded as being lost five years previously. After killing an LSF officer in self-defense, Trent and Juni are forced to flee Manhattan, whereupon they are attacked in orbit. However, Walker arrives in his ship, accompanied by LSF officer Michael King, Walker and a group of fighters designated Lambda Wing, and destroys the Battleship Unity so as to allow Trent and Zane to flee to Benford Research Station in the Badlands of the New York system.
In an assault on Zone-21, Walker sacrifices himself and his ship in order to destroy two Liberty Battleships guarding the Jump Gate from New York to the Alaska system. As a result, Trent, Juni and the Order ships are able to escape the Alaska system after they rescued President Jacobi from the clutches of the Nomad-possessed Rheinland Chancellor Niemann.