Elektra Natchios (Jennifer Garner), not knowing that her lover Matt Murdock (Ben Affleck) is Daredevil, blames Daredevil for her father's death, catches up to him, and severely wounds him in the shoulder. Before administering the kill, she removes his mask and realizes the truth. Unfortunately, the injury prevents Murdock from helping her when the real killer, Bullseye (Colin Farrell) shows up, and (in a scene amazingly faithful to the comic book original) kills her with her own weapons. Murdock regains enough strength to fight Bullseye, who, in his cockiness, reveals that The Kingpin is Wilson Fisk (Michael Clark Duncan). Murdock defeats Bullseye, throwing him through a stained glass window to *SPLAT* on Ben Urich's (Joe Pantoliano's) windshield several floors below. He then moves on to Fisk's office, and defeats Fisk in a nasty fight. And even though Fisk has killed the two people Murdock loved the most (and learned his secret identity), he leaves Fisk, kneecapped, for the police and the criminal justice system.
P.S. As the film is closing, Murdock finds a good luck charm similar to Elektra's, but written in Braille, on the roof of his brownstone. The obvious implication is that she'll be back in the sequel (don't worry, it's not cheating; they did it in the comics as well).
P.P.S. In a bonus clip during the end credits, we see that Bullseye survived his fall. Even bandaged and in traction, his aim is still deadly.