I found this on wiki so if you dont want to read a *SPOILER* don't go any further down.
In the first act, a young, innocent village maiden named Giselle is in love with a man she knows only as Loys. In reality, the man is Albrecht, a nobleman disguised as a peasant. When Giselle discovers the deceit, she is inconsolable and goes mad, then dies of a broken heart. In the second act, her undying love for Albrecht saves him from the wicked magic of the wilis, vampiric ghosts of betrothed girls who have died before their wedding day. Though their leader, Myrtha, Queen of the Wilis, forces Albrecht to dance again and again, Giselle intervenes long enough to spare his strength and allow him to survive until the dawn. At sunrise, wilis must return to their grave; Giselle must return as well but not before showing Albrecht that she forgives him for his treachery. The two pledge their love to each other and she descends back into her grave, perhaps a wili no longer.