Lisa's wedding
On a renaissance fair, Lisa walks into a gypsy's tent who proposes to tell her her future.  At first cynical, Lisa decides to listen anyway, as the gypsy tells her the story of her first loves.  When she comes out of the tent and looks back she sees that it has disappeared...
At university Lisa needs to borrow a book from the library, to find that the last copy of it has already been lent out to a Hugh Parkfield, a rich English man, who has previously already been bugging Lisa by taking the last of things.  They decide to read the book together to see who reads the fastest, but by the end skip the reading and start making out.  Hugh invites her to stay at his parents' country estate, where he proposes to her, and she accepts, gladly.  They decide to hold the wedding in Springfield, which means Hugh and his family get to meet Lisa’s family, which makes her slightly nervous, even more so because his family seems to not like the Simpsons.  As it comes apparent to Lisa that Hugh will never accept her family the way they are, and that he intends to take her away from it all to never return, Lisa calls of the wedding and chooses for her family.

Bart to the Future
In an Indian casino, Bart gets pulled aside because he is too young to be there.  In the owner's office, Bart is told to look into the fire, where his future is shown to him.  A future that Bart actually quite likes, even though it was meant to make him change his ways.
Bart and Ralph Wiggum share a house/slump together and are trying to make it big in the music world, but so far things aren't going too good.  When he finally does get a gig, his amp's in hock, so he needs to get some money.  After Homer and Ned turn him down, he decides to go to his sister, the president elect of the United States.  She doesn't at all like her presence at the White House, but has no chance but to tolerate it at least for a while.  While Homer goes in search of Lincoln's gold (which is a metaphor, btw), Lisa decides to raise taxes to help the US out of it's debt, but when she's addressing the public Bart comes in to interrupt her, so she sends Bart of to Camp David, where he meets the ghost of President Carter, who tells him to go back.  Back at the White House Bart manages to fence of an argument that could have resulted into another World War, and to pay him back, Lisa legalizes 'it'.

Future-Drama
While fighting, Lisa and Bart roll into professor Frink's house.  he then shows them what their future will be like as brother and sister, Lisa being a bit more interested in it than Bart.  In the end we learn that the brother-sister bond will prevail, and Bart will find true love, but he'll also die one minute after.
On their prom night, which is the same night since Lisa is graduating 2 years early, Bart proposes to his girlfriend Jenda.  She doesn't accept and they break up because Bart has no future planned.  Lisa went to the prom with a muscular Milhouse, mainly because she owed him for saving her life, but they also break up.  Bart is really sad about losing Jenda, so he takes a job at the Kwik-E-Mart to show he has a future and win her back.  He has to deliver some groceries to Mr. Burns who is at that moment being robbed, and saves the old man's life.  Out of gratitude, Mr. burns gives him the scholarship that was meant to go to Lisa.  Bart gets Jenda back because of it, and Lisa is left with no prospects for the future, except ending with Milhouse.  Bart and Jenda walk by professor Frink's house where Bart takes a look at the machine that shows the future, and learns from it that Lisa will be miserable with Milhouse, so he pulls her away from him at the moment that they would share a kiss, and lets her know that he is giving the Yale scholarship back to her.
In the mean time Homer and Marge have split up because Homer bought an underwater house, and Marge starts something with Krusty.  Homer is jealous, and picks a fight, but ends up beaten up by the clown.  Strangely enough this wins Marge back, and she realises that an underwater house can be romantic.