Someone's been experimenting on mutants.
Whether they've escaped or someone's letting them loose into the world for their own purposes, a series of incidents slowly reveals mutants who have been altered through surgery and strange science. Some have been made more powerful. Some have been transmogrified into something far, far from human. Some are just victims, pitiable things begging for release.
They all agree on a single detail - one man is behind it all and that man has plans for mutantkind. And some members of mutantkind in particular.
Sometimes evolution needs a push.