What exactly happened to Agent Callen's (Chris O'Donnell) sister on NCIS: Los Angeles? The answer is a lot to unpack, and just one chapter of Callen's complicated backstory.

Agent "G" Callen has been an NCIS: LA mainstay since Season 1 aired in 2009. While Callen had extensive experience in working with special organizations like the FBI and the CIA before joining the NCIS Office of Special Projects in Los Angeles as the special agent in charge, one mystery had always plagued him: Who was his family? What happened in his past? Callen didn't even know what the "G" in his name stood for for years because he grew up in foster care.

One of the first mysteries Callen uncovers regarding his past is the question of what happened to the sister he initially forgot that he had. Here's what really happened to young Amy Callen, Agent Callen's sister.

What happened to Callen's sister?

Way back in the Season 1 NCIS: LA finale, Callen remembered that he had a sister named Amy Callen. The two were separated and sent to different foster homes. Amy ended up at an orphanage where she went out to a river one night with her friend Hannah Lawson. Amy tragically ended up drowning in the river that fateful night. Panicked, Hannah returned to the orphanage and took Amy's name.

After Callen discovers Hannah (Jacqueline McKenzie) is still alive, he initially assumes it's his long-lost sister. Hannah ends up telling him the truth about the real Amy's death. The reason that Amy and Callen ended up in foster care in the first place was due to the fact that their mother Clara was killed years after fleeing to America from Romania. Clara's father had initiated a blood feud in Romania with the Comescu family that forced Clara to flee her home. Clara ended up working for the CIA.

In yet another chapter of Callen's sad backstory, Clara was assassinated on a beach by a member of the Comescu family while seeking safety for the young Callen and his sister Amy. Clara had been waiting on said beach for Henrietta Lange (Linda Hunt), or "Hetty," to take Callen and his sister to safety.

Just when NCIS: LA fans thought Callen's backstory couldn't get any more complicated, Agent Callen delved further into his complicated past during the Season 13 NCIS: LA premiere with a little unauthorized search into Hetty's files. He discovered that he was, in fact, "Subject 17," one of several children recruited specifically to join the CIA. Hetty returned to NCIS: LA in Season 13, Episode 1 in a glorious fashion, ultimately confirming Callen's suspicions that he was Subject 17.

What does that mean for the future of NCIS: LA moving forward? Agent Hanna (LL Cool J) warns Callen in the Season 13 promo, "You go down this rabbit hole, G, you may not like what you find." Hopefully Callen's quest to continue understanding his past won't put him in too much peril moving forward.

Catch new episodes of NCIS: Los Angeles on Sundays at 9 p.m. ET on CBS.