NCIS: Los Angeles has begun filming on their 100 th episode and, while the cast celebrates their achivement, the writers have laid out an exciting plot for viewers.

Bearing the title "Reznikov, M," the milestone installment will take a peak at G. Callen's (Chirs O'Donnell) past. The producer has been teasing the reveal of more information on the agent's mysterious past for weeks and it appears that clues may culminate in the 100 th episode.

"Promised spoiler: GCallen discovers more of his history, in a big way EP.504-100," tweeted executive producer John Peter Kousakis.

Written by series creator Shane Brennan and directed by Tony Wharmby, production for episode 504 kicked off on Friday with a table read and a celebration.

"Our 100th episode starts shooting today. 100 has three digits. Not two. For that reason, we will be having cake at work today," tweeted Barrett Foa who portrays Eric in the hit CBS show.

The entire cast and crew gathered together, including Peter Cambor who occasionally plays operational psychologist Nate Getz.

"I had started a diet this week, but I thought.... Ah, what the hell! #NCISLA Congrats to the cast and crew," wrote Cambor along with the picture of the production's half-eaten cake.

Check out the gallery above for some photos of the celebration and the start of filming.

NCIS: LA returns on Sept. 24.

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NCIS: LA, NCIS: Los Angeles