This is one of those movies that took a minute for me to get into. At first I thought it was cheesy and the characters were idiots. Though not entirely untrue, there is a charm and complexity to Bill and Ted's Excellent adventure that upon subsequent viewings has really grown on me.
When you really think about this movie it is really about maintaining its own paradox. Rufus, played by the amazing George Carlin, is sort of a time angel who explains the importance of Bill and Ted passing their "history of everything" class. It's not specifically stated but they need to pass the class because a utopia society has sprung from the teachings of Bill and Ted. In watching the movie though I noticed it's not so much passing this class that creates this society it is Rufus's meddling. By sending Bill and Ted through time to meet all these world leaders and showing them how to "be excellent to each other" and "party on" they change history and time itself, steering the future society to the utopia shown in the film.
I think the underlying plot of this film is that Rufus discovered that his meddling has inadvertently created a perfected society and is now ensuring that those events happen. It's rather paradoxical and shouldn't be analyzed too heavily, but for a film that has so much fun in its ridiculousness its a pretty interesting big picture as far as time travel films go.
Imagine the unseen movie where Rufus has been trying to fix the timeline for years after some sort of time travel accident and discovers that the secret to a perfect society was two idiots from San Dimas California passing a history class.