Michael shows up to work with a fake mustache. When the mustache falls off, everyone becomes well aware of the cold sore on his lip. Michael freaks out when Kevin tells him it's an STD and he needs to be tested. Andy wants everybody to chill out on the herpes talk.
Dwight convinces Michael to call his ex-girlfriends and track down the one who gave him herpes. His first call is to Donna. She denies giving him the disease and hangs up when he accuses her husband of having it. The next call is to Holly. Michael gets sentimental, missing her and hangs up.
Michael decides to visit Jan personally at her office. She admits that life has been moving fast for her and she's taking on the role of supermom with Astrid. Michael freezes up when he has to tell Jan the bad news about his cold sore. (game skittle to Jan's Trivia) Jan thinks Michael's there to talk what went wrong in their relationship.
Andy summons the everyone to the conference room for a pizza party and a frank discussion about STDs, after seeing the way the office treated Meredith when she revealed she had herpes. Andy makes list of the pros and cons of having herpes. Only problem, more of the coworkers are naming pros than cons.
Andy uses his speech to fish for more details about Erin and Gabe's relationship, including if they're having sex. When they don't seem to deny they're intimate, Andy's temper gets the best of him and he throws a pizza against the wall. Gabe is forced to reprimand Andy for his behavior.
After Michael's visit with Pam's mom Helen goes badly, he tracks down Carol, his ex- girlfriend and realtor. He tells Carol he has herpes. When she sees his cold sore, she tells him to see a doctor and the two begin to rehash what went wrong in their relationship, starting with Michael's tendency to over-romanticize ever detail.
When Michael returns to the office he calls Holly back. He tells her that after visiting all of his ex-girlfriends, he realizes that Holly really was the one he loved the most. He leaves a voicemail telling her he's still confused over why she accused him of taking their relationship too seriously, because he knows they were the real thing. And she might want to see a doctor for herpes.