This is a sequel to the 1994 miniseries that took place in an Apocalypse future timeline. Instead of launching Jean and Scott into the future, it sends them back to 1859, where they get a chance to kill baby Apocalypse. Or at least prevent Nathan Essex from becoming Mister Sinister. Or something like that.
Essex is portrayed as a scientist working on Darwinian theory who becomes a victim of circumstance, corrupted by Apocalypse.
Essex’s theories on evolution are not accepted by his peers. Darwin himself does not approve.
Not sure why Mister Sinister needed to be rehabilitated, and I don’t love all the connections between every point in time and Apocalypse, but if you’re a fan of Sinister, then this extended origin is probably something you’ll want to be familiar with.