It's origin story time on NCIS: Los Angeles, and fans will finally learn how one of the series' most prolific partnerships came into being.

Mysterious 'Kill House' Character Joins Nate and Danny Trejo In 'Tuhon', Will 'The Box' Return On 'NCIS: LA'?

While Deeks and Kensi's became partners before viewers' eyes, Callen (Chris O'Donnell) and Sam (LL Cool J) rolled onto the scene as an already well-oiled machine. A new preview for next Tuesday's "Tuhon," however, reveals that the duo received their start courtesy of an assassin (Danny Trejo) - the very same assassin who will resurface as a suspect in the recent death of a diplomat (see video below).

"We don't flash back and see them [during their first case], but we discover how they first met. It involved a character called Tuhon, played by Danny Trejo, and an unresolved case," explained series creator Shane Brennan, according to TV Line. "Here we are, six years later, and Tuhon's back in circulation, back on their radar, and we discover a whole lot more about that character than they ever knew when they first went after him. What we learn is that he has a very close connection to Hetty."

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Penned by David Kalstein ("The Frozen Lake," "Kill House"), "Tuhon" will send Callen and Sam south of the boarder while Deeks pairs up with the oft-absent Nate Getz.

Considering NCIS: LA season 5 has been referred to as the season of Deeks and Kensi, the upcoming episode will be a refreshing break for fans of the Callen/Sam dynamic. Although Callen played out his battle with Janvier (Christopher Lambert) in the season premiere and the 100th episode put a laser focus on his mysterious past, the largest role the duo has recently had was during "Merry Evasion."

Do you prefer the Callen/Sam storyline or the Deeks/Kensi plot? Sound off in the poll below!

NCIS: Los Angeles airs Tuesday at 9 p.m. ET on CBS.