Ferris Bueller is taking another day off, this time taking his new girlfriend, (who also holds the title new girl in town) Ember McFly. One problem, Ember's brother, Marty, would never let her.
(Spoilers: she goes anyway)
While the three are all arguing, they accidentally set the DeLorean to the 1960s in Tulsa, Oklahoma, where they meet the greasers.
When they get back, Ember and Ferris's curiosity gets the best of them as they follow a group of young friends to find "hidden treasure" with a map that leads them under the school, and Marty reluctantly follows as well.
To make a long story short, that whole endeavor landed them all in detention.