I'd do the smart thing. First off, I'd alert everyone I could online. Every community from webcomics to general media, claiming to ask for advice and any help they may have. Next, I find a good lawyer who will work pro bono because he can see from the evidence it's a pretty open and shut case.
But, before I do that, I find a way to contact the people and make them an offer. 30% gross profit on whatever the liscence makes and I don't sue, plus $20 million up front to keep my mouth shut, which shall be under the guise of buying the rights.
Of course, that's only for someone big like Fox, Warner Bros. or Disney. For smaller companies I'd probably ask for less up front.
Lol, good luck with that offer. Writers are poor for a reason!
But anyway, my comic is completely un-copy-able. I'd struggle to come up with a plot myself, though I certainly will do so at some point.