-Freddie Hart was McGarrett's best friend, as well as his partner on the fateful mission to North Korea to retrieve Anton Hess--the mission depicted in the show's pilot episode.
-Freddie died in North Korea, but his body remained there for three years. The North Koreans offer the body of another man to the U.S. military, claiming that it is Freddie's, but McGarrett knows that it is not Freddie--and that he must travel to North Korea and retrieve the body himself.
-McGarrett and Catherine sneak into North Korea, tangling with the same war lords who murdered Freddie. They uncover Freddie's body, kill the men who killed him, and bring Freddie home.