There are a few reasons why this was Twentieth Century Fox's highest grossing movie for 2010:
It has stars who we all know (Tom Cruise and Cameron Diaz),
there is a lot of action, and
it is really funny!
This is the type of movie that you want to rent and watch on a big screen with a group of friends. There are enough places without dialog, so the viewers can chat and pass around food.
Plot (while being careful not to spoil):
An apparent chance meeting in the Wichita airport and subsequent airline flight to Boston turns June Haven's (Cameron Diaz) world upside down. June becomes entwined in a plot with both her life, and the security of critical United States secrets, at risk.
June struggles to decide whether she should trust the able and good looking Roy Miller (Tom Cruise), a government agent who claims he has been unjustly accused of malfeasance. June's decision to trust Roy seems to be more based on emotion and Roy's charisma, than on the apparent facts.
Roy and June head off to help somebody else who is in danger. This results in some serious altercations with various foes and major globetrotting for June (who never even had a passport).
Knight and Day has action/violence, tender moments, and a lot of laughs!
I rented this from my nearby Redbox for $1 plus tax.