Knight and Day follows the adventures of Roy Miller (Tom Cruise) - whose real surname is Knight - and June Havens (Cameron Diaz). After literally bumping into Miller twice at the airport on the way home from Wichita to pick up car parts, Havens is told she has been bumped to a later flight. Meanwhile, Agent Fitzgerald (Peter Saarsgard), believing Havens is working with Miller, puts her back on the plane. Completely taken with Miller, Havens goes to the restroom; and the remaining passengers all attack Miller. Miller kills them all and props the bodies up in their seats. Havens emerges from the restroom, where she has been freshening up in the hopes of a sexual encounter with Miller. Having heard nothing, she is totally unaware of the attack on him. After a forward moment where Havens kisses Miller, he calmly informs her that everyone on the plane is dead, including the pilots. Believing it a joke, Havens plays along until Miller enters the cockpit and the bodies of the passengers begin to shift around the plane. Crashing into a corn field, Miller delivers Havens home after drugging her and explaining that other agents will visit and that, if they constantly repeat that she is being taken somewhere "safe and secure", they are planning to kill or imprison her.
Waking up at home, Havens struggles through a day fitting a bridesmaids dress for her sister April's (Maggie Grace) wedding, and is shocked to learn her sister would like to sell their father's car, which Havens had planned on finishing as a wedding present. Havens is then picked up by a group of intelligence agents. Miller arrives and, through a long gunfight on the highway, kills several agents and reclaims Havens.
Havens flees at the first opportunity and contacts Rodney (Marc Blucas), a firefighter and former boyfriend. Believing Havens is merely stressed and is playing out a fantasy, Rodney tries to comfort her until Miller arrives and kidnaps Havens, handcuffing her and shooting Rodney in a specific non-vital area ,telling him this will all make him a hero and virtually guarantee his promotion to lieutenant.
Miller explains that Havens is safer with him; and Havens agrees to follow him as they go to pick up Simon Feck (Paul Dano), a genius inventor who created a perpetual energy battery called the Zephyr. Miller arrives at his safe house where he left Feck, only to find him gone, and the two are ambushed by men belonging to Antonio (Jordi Mollà), a Spanish arms dealer. After Miller again drugs Havens, she drifts in and out of consciousness between their capture and escape from Antonio's men, and Miller brings her to an island that is off the grid, which Miller uses as a safehouse. Accepting a call from her sister after leaving in frustration, Havens accidentally leads Antonio's group straight to the hideaway. They try to kill Miller and Havens with a remote drone.
Again knocking out Havens, Miller transports them to a train heading through the Alps, which Feck left as a message in code for Miller. Havens, missing a message from Miller, leaves to get dinner and encounters Danny (Rich Manley), a German assassin. Using tricks learned from Miller, Havens manages to survive, and Danny is eventually knocked out of a window by Miller and hit and killed by a train coming from the opposite direction.
Miller, after leaving Havens and Feck in a hotel in Salzburg, heads to a meeting with a mysterious beautiful woman with whom he is apparently willing to make a deal, and is followed by Havens, who is heartbroken to learn that Miller is apparently planning on selling the Zephyr to Antonio. After being picked up by the CIA and meeting the director of counter-intel, she uses a pen transmitter to notify them when Miller returns with the Zephyr, which is showing signs of overheating. After leading the CIA agents on a chase, Miller is apparently shot and falls into a canal.
Returning home, Havens heads to an address she remembered from Miller's iPhone, where she finds his parents and learns that his real name is Matthew Knight. They believe their son, a former Army sergeant and Eagle Scout, is dead; but they are fabulously wealthy from winning lotteries and sweepstakes they don't remember entering. Leaving a message that she has the Zephyr, she is captured by Antonio's men and taken back to Spain. She is drugged with truth serum before being rescued by Miller, who was tracking Fitzgerald, who was delivering Feck to Antonio.
Chasing Fitzgerald, Miller saves Feck from a bullet wound after handing over the Zephyr in a small satchel. Feck comments that the battery is unstable. The battery explodes, killing Fitzgerald; and Miller collapses from a gunshot wound. After waking in the hospital, Miller receives an apology from the director (Viola Davis), who tells him that Havens has returned home and claims that Miller can't be distracted. Miller is later given medication by a nurse, who turns out to be Havens. After Havens breaks Miller out of the hospital, he awakes as she drives away in the rebuilt GTO that belonged to Havens's father. After Miller asking what day it is, Havens kisses him and says it's someday. This is a reference from the start of the movie that they both have things they want to do someday, and Havens begins to drive towards Cape Horn. The last scene shows that Roy's mom was telling her husband that he bought two tickets to Cape Horn even though he did not. She did not know that he did not buy the tickets.This scene inferred that Roy was going to meet his parents in Cape Horn.
