Doctor Judith Mossman is a theoretical physicist. After the Combine invasion she became a member of the resistance, and helped develop their teleportation technology. While working at Black Mesa East with Dr. Eli Vance and his daughter Alyx, she was also a spy for the Combine. She leaked information on uni-dimensional teleportation to Dr.Breen. The Combine could have attacked and taken over Black Mesa East at anytime, but because of Dr. Mossman and the information she had they did not.
Mossman planned on turning in Gordon Freeman to Breen herself, but the Combine followed Gordon to the base and were ambushed before she could set things up. After the attack, she went to Nova Prospekt to work on the Combine’s new local teleporter. It’s here that Alyx and Gordon learn that Mossman is a spy and plan to capture her while rescuing Eli Vance, who was captured in the Black Mesa East Attack. When cornered by the two, Mossman explains that she had Eli’s best intentions at heart. She was convinced by Breen that the resistance and Eli’s chances where doomed and there best chance was to side with the Combine.
After an argument at Nova Prospekt, Alyx forces Mossman to help her escape with Gordon and her father. When they reach the prototype teleporter she had been working on, Mossman distracts Alyx, and transports herself and Eli to the Combine Citadel in City 17. At Breen’s office in the citadel, Mossman realizes how wrong she was about everything. She learned that Breen was going to send Alyx and Eli to the Combine Overworld. Mossman turns on Breen and threatens him with Alyx's Electronic Multi-tool and seals the main door to Breen's office. Breen manages to escape, so Alyx and Gordon chase after him while Mossman helps Eli escape the citadel.
Half-Life 2: Episode 1 Dr. Mossman appears in a video recording stored in the Citadel. In the video she is dressed in winter clothing and is sending a message about the discovery she made. As she is about to give the location of the project, Combine soldiers blast through the door and force her to retreat deeper into the building.
Half-Life 2: Episode 2 At the White Forest Base, Eli Vance and Isaac Kleiner watch the Mossman video recording Alyx and Gordon brought them. They believe Judith Mossman's team found the Aperture Science research vessel Borealis. The Borealis suddenly disappeared one day taking the crew, and even part of the dock with it.