At the end of season 6, bipolar former CIA agent Carrie Mathison (expertly played by Claire Danes) and Peter Quinn (played by Rupert Friend) helped foil the assassination attempt on President Keane (played by Elizabeth Marvel), with Quinn paying the ultimate price, giving his life to protect the President.
Which in hindsight probably wasn't worth it since Carrie realised in the season 6 finale that President Keane is actually a total villain and had been very busy imprisoning a huge number of members of the intelligence community.
Cut to the tension-filled, action-packed season 7 trailer, Carrie says, "The country is in free fall," and she's given herself the task of getting it back in track which means taking down the President.