The Board is forcing Dr. Kelso to retire because of his age. Carla & Elliot feel guilty and decide to help him out by getting everyone in the hospital to speak to the Board on his behalf.
Meanwhile J.D. and Turk diagnose a patient that stumps Dr. Cox (although the only reason they know is that they left the urine sample out in the sun).
While everyone else is occupied Kelso bonds with one of the interns and tells stories about Sacred Heart...he truly did love the place. In the end everything makes sense: JD comes clean to Dr. Cox, and Kelso gets to leave on his own terms.
Surprisingly I'm sad that Kelso's gone, who else will have two thumbs and not give a crap? And who else will get the muffins for life?