Help! Need to design a larp plotline by 12.00 tommorow

Right...

I've volunteered to create a larping plotline for me and two, although possibly three or four, friends, all of whom are experienced at larping and roleplaying. We want to do a non-costume larp centred around a trip from our hometown (Exeter) to a beach town (Dawlish Warren), and I'm supposed to create a mystery plotline. It should ideally be about three to five hours to play, with a mysterious plotline that is figured out by them over the course of the game. We're imaginative, but we like to at least somewhat root it in reality, so a contemporary setting (or a "what the hell is this old-fashioned/futuristic world" idea.)

We're all experienced enough to carry off something complicated and I can rustle up virtually any easy-to-make props an idea would need. We have several sets of pre-made characters for almost any tech level, and we can make some up on the fly if needs be. Has anyone got an idea for a cool plotline? You'd be saving my life!! :D