It's been building for some time now. Amidst the controversy that was the reveal of Steve Rogers (Captain America) being an agent of Hydra, the story began. Secret Empire is a story told across several Marvel titles but culminating in this new series. Here's a teaser trailer for the series along with some comments from writer Nick Spencer:

The plot is described as follows:

There's a lot of things happening in this series, least of all is Cap being in charge of Hydra and holding Thor's hammer! Secret Empire #0 and Secret Empire #1 are both available now!