Chase and Dr. Kelly make their way down to the ER. Chase tells her to start "thinking" like House and see whatever's wrong with Billy as something related to whatever's wrong with Margaret. Dr. Kelly says that could be it, but it could also just be a coincidence, thereby not thinking like House at all. In the end, what's wrong with Billy is that he got his face smashed in. He says he found an address in Margaret's book for a guy named Carl in Brooklyn and went up there to confront him, assuming he was the evil ex. He then learned a valuable lesson about why you shouldn't confront someone from Brooklyn. Carl ended up not pressing charges, but he convinced Billy that he was never married to Margaret, had no what she was talking about, and only knew her because they used to work together. And then Billy apparently drove back home to New Jersey while bleeding from various face wounds and went to the PPTH ER. Meanwhile, Foreman calls Chase to inform him that Margaret is now running a fever, which means she wasn't poisoned after all.
Back in the meeting room, House entertains the group with a joke about Carlsbad Caverns and how Billy would try to beat that up if he found it in his wife's address book, then moves onto the slideshow portion of the differential diagnosis of Chase's mental issues. "Meet your doppelganger!" House says to Dr. Kelly, only to show her a slide of a foot with something horrible going on. House apologizes for putting the wrong slide on, and switches it to the correct one: a picture of one very young Chase and an older woman who must be his mother. And whose face has been photoshopped to resemble Dr. Kelly's. "That's baby Chase and his mum! That's adorable!" Taub says. It is, until you remember that Chase's mother drank herself to death when Chase was still a child and then it becomes kind of sad.