What the heck??!

[Matchbox01]

I have a 2001 Porsche Boxster with 80K miles on it. 3d owner. I've been having trouble with the manual transmission. End of October I started getting clutch slippage, or transmission slippage - high engine revs with poor acceleration after shifting, slow at slipping into gear. Finally, the whole program failed one night while driving on the highway. Smoke, no ability to get into any gear, lots of play in the gear shift lever. Clutch pedal responsive. I was stuck on the side of the road with no engagement with the transmission at all, like in neutral. Had it towed home. Smelled nasty for about a week. Small dark leak under the car - nothing massive. What happened? Is this the clutch? The transmission? Or the drive Train? Some combination? I'm trying to figure out what to do next, so a point in the right direction would be very much appreciated.