A Possum on Chemo is the 14th episode of the ninth season of Two and a Half Men. It has aired on January 16, 2012.
When Walden and Zoey are heading out for dinner one night, they spot a lone Alan taking profile pictures for an online dating site. Walden pities him and invites him to join them for dinner, an action that seemingly annoys Zoey. At the restaurant, Walden spots Lyndsey, Alan's former girlfriend. Alan assumes that she is as lonely as he is, and starts to talk to her. Then, a young and rich man named Dylan shows up, turning out to be her date. Alan is confused, but gains new hope when a stoned Eldridge tells him that Dylan is gay and that Lyndsey misses him. Alan immediately heads out to reunite with his former girlfriend, just as Eldridge reveals to Jake that he lied just to get his father to leave. Before he can ring the bell, Alan hears Lyndsey and Dylan having sex and being devastated, leaves. To his surprise, Lyndsey calls him the next day, telling him that she wants to get together. Later, while making out, Lyndsey tells him that it got too hard for her to keep up with a young man like Dylan, as she couldn't let him see her without makeup or use the toilet in front of him. She then reveals that she is happy to be with Alan, as he cannot find someone better and is more than happy with her. Alan seems to be all the more confused after hearing this.
Meanwhile, Walden finds out that Zoey dislikes his appearance and wants him to shave and cut his hair. Walden is initially reluctant and is assured in his position by Alan. When he asks Zoey why she didn't say something earlier, she reveals that an important business party, where Walden cannot show up unkempt, is coming up. Walden finally agrees (with Berta being revealed as a hairdresser), but then changes his mind again when Alan tells him about the things Judith made him do. When he shows up at Zoey's unchanged, they start a fight and Walden runs off after mentioning that he has hopelessly fallen in love with her. Zoey later stops by to apologise, where she is greeted by a groomed Walden, who tells her that he wanted to shave and cut his hair when no one was nagging him to do it.
At the end of the show, Zoey is seen coming out of Alan's bathroom, thanking Alan for letting her use his bathroom as she doesn't want Walden to hear any of her toilet noises, with an understanding Lyndsey agreeing with her and recalling a story about a bathroom incident he had while being with Dylan.