Its entertaining. There is a plot, and here it is:
The Persians, with the help of a Spartan traitor, bribe the Spartan priests to withold their blessing for military action. Leonitus and 300 spartan warriors go to hold back the persian army, which is illegal without the blessing of the priests. Leonitus's wife must try to convince the council to go to war while the 300 soldiers hold back the massive persian army.
BAM. I'd say you have yourself a plot there. It's not an incredibly deep plot, (and yeah theres lots of historical inaccuracies) but who cars, just watch the fuckin movie