House carols his way into the lab, where Foreman tells him to leave them alone to run their tests in time for Christmas. House says that there's no need -- they just have to shoot Maggie up with an anti-psychotic so that he can perform a Christmas miracle.
Risperidone isn't just any anti-psychotic -- it's one that makes breast tissue swell. And breast tissue isn't just any tissue -- it happens to cover the fetus when it's in the womb, receding back to the "fun places" as the fetus develops. Sometimes, though, the breast tissue doesn't recede all the way, and gets left behind in non-breast places. The swelling from the risperidone should help them find any wanderin' breast tissue if that's the case here. And if that is the case, maybe that breast tissue has cancer, and that's what's causing all of Maggie's problems. Well, that's a pretty big stretch, but since Wilson already proclaimed Maggie cancer-free and we've seen cancer where there shouldn't be happen before, I'm guessing House is gonna find some cancer. Lo and behold, House finds a hugely swollen red lump behind Maggie's knee. Damn, that risperidone works fast! And well! Flat-chested women should take this -- they'll be Ds in no time! Of course, Agnes doesn't think the lump is breast tissue, so House proves it by sticking a needle in the lump and withdrawing a syringeful of white liquid. Ew. House explains that risperidone also makes breasts lactate. The white liquid is milk. Again, risperidone is a miracle drug! Woman who can't breastfeed should try it -- they'll be pumping out five-gallon jugs daily! House tells Jane to open her mouth, and then he shoots the breast milk inside it. He says she's had it before, which she actually hasn't now that we know she's adopted. He knows it, too. One thing I know is that squirting knee breast milk into a child's mouth is fucking gross and so inappropriate. Sometimes it's funny when House is inappropriate. This episode was not funny. But the good news is, Mom will survive her knee breast cancer (that somehow causes red eyes and blindness and stone bones) just in time for a month-late Christmas. Maggie reacts to this by not telling Jane the truth about her biological mother.
And it wouldn't be Christmas (and...it's not, but whatever) without a Christmas-music montage, where we get to see Cameron for one second at the holiday party. She and Chase hang out with their replacements and tell them about all the lines they used to have. House just walks by them like a big old limping Scrooge, deigning only to give Foreman a slow-motion nod on his way out.