I'd say the OP contained the premise of a story or the basic idea for starting one, or yes, a hook, but I agree with Axiom that it's not really a plot, if you're approaching 'plot' in a more specific literary sort of way. The blurb on a book jacket isn't a plot either, for that matter.
Since in this one it's promised that 'all will be explained in time' I do hope that means they've got one in mind, though.
It really is a semantics thing and so I don't see the point of it being brought up in a thread like this, but IIRC @Bucky had a nice simple explanation of the difference between plot and just the events of a story, seeing how the latter could be any kind of pointless or random shit. 'Plot' requires a certain amount of purpose and cohesion.