G Callen's past has come back to rear it's mysterious head on tonight's episode of NCIS: Los Angeles.

The agent's history is notoriously cryptic. After growing up in 38 different foster homes, Callen finally uncovered the story behind his mother's death during season 2 and 3. However, digging into the past meant dropping himself into the middle of a generations-old family feud with the Comescus. As the series' 100th episode begins, the crime family will be back on the agency's radar.

"It's one thing to go to Romania and poke the hornet's nest," said series creator Shane Brennan, according to TV Line. "It's another thing to have the hornet's nest drop into your lap in L.A."

The episode promises to introduce viewers to Callen's father, or at least a man who claims to be his relation.

"There is a line in the episode where a particular man says, 'I am your father,'" Brennan told TV Line. "After the [opening] credits, we go back eight hours and you'll see the day unfold, taking us back up to that point. The first minute of the show is very shocking - two of our people, we don't know if they're dead or alive."

A new sneak peek (video) shows an unresponsive Sam (LL Cool J) trapped in a car fire next to Assistant Director Granger (Miguel Ferrer), who appears to be shot in the chest. As an unidentified gunman walks past the scene, he drags away an injured Callen who then comes face to face with his alleged father.

"If you go back to the beginning of Season 2, the very first episode, there's a moment in there that I set up and we in fact we deliver in the hundredth episode. What was really interesting in looking at the hundredth episode, because it's obviously Callen-centric, was the amount of his backstory that we covered in those other 75 episodes," Brennan said, explaining to TV Line exactly how long this storyline has been in development. "Everyone who's seen it, the first thing they say - every time and without exception - is, "This is Chris O'Donnell's best work." He is fantastic in this episode. It's very powerful and you will be affected by the ending. You really will."

NCIS: LA's 100th episode, "Reznikov, N," will air tonight at 9 p.m. ET on CBS.

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NCIS: LA, NCIS: Los Angeles, CBS, Television