S3, E23: When Charlie finds out that his ex-girlfriend, Mia (recurring guest star Emmanuelle Vaugier), is in town with her dance troupe and wants him to come to her performance, he opts not to go and tries to behave as if he were no longer attracted to her. After surprising Alan--and himself--by showing up at the dance performance, Charlie gets another surprise when Mia explains that she wants something personal from him: his sperm. Shockingly, commitment-phobic Charlie responds by asking Mia for something as well.