Joyce Kim was mentioned in the pilot (S1E01), "The Codpiece Topology" (S2E02) and "The White Asparagus Triangulation" (S2E09). She appears in flashbacks shown during "The Staircase Implementation" (S3E22). In 2003, Leonard was working on rocket fuel at the time under a DARPA contract and Joyce Kim was a spy attempting to obtain information on his work by seducing him and report it back to her government. Luckily, Sheldon unknowingly stopped her, after he intruded on their fun, claiming Leonard had violated the roommate agreement. Leonard agreed to give Sheldon a 12 hour notice before having guests over, but he hadn't known Joyce Kim for 12 hours before he had her over. It is unknown when she will appear again, if ever.
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