NCIS: Los Angeles is home to numerous characters with complicated and confusing backgrounds, but none have quite as mysterious a past as Agent Callen (Chris O'Donnell).

When the CBS drama returns for its fifth season, viewers get to delve further into the puzzle of G. Callen's hazy history.

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"GCallen spoiler...more family history revealed early in S5," tweeted executive producer John Peter Kousakis. "More on all cast throughout S5."

As the series gears up for their 100th episode, the landmark installment promises to provide a windfall of information regarding Callen. Titled "Reznikov, N" (previously referred to as "Reznikov, M"), the episode will see Callen team up with an old enemy while also revealing the identity of the title character.

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"Anyone who has followed the Callen story, in his search for his identity to find out who he really is - to fill in the puzzle, he has that tea tin that sits on his mantelpiece and has little jigsaw puzzle pieces of his life in there - he gets to put another very, very significant piece of the jigsaw puzzle into that tea tin," said series creator Shane Brennan, according to Xfinity.

NCIS: LA Cast Films 100th Episode For Season 5  

As far as titles go, similar names have been given to other watershed episodes, including season 1's "Callen, G," and season 3's "Lange, H," and "Blye, K." The 100th episode, slated to air as the fourth installment of season 5, promises to reward fans for tuning in to the show's fifth year.

NCIS: LA returns to CBS on Sept. 24.

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