Chris O'Donnell

NCIS: Los Angeles executive producer Shane Brennan promises that Tuesday's episode will reveal more about the mysterious past of Chris O'Donnell's Agent Callen than the first two seasons combined.

Fall Preview: Get scoop on all your favorite returning shows

"This is a moment that we've been planning for 50-odd episodes," Brennan tells TVGuide.com. "In the first four or five minutes, you will learn an enormous amount of detail about who Callen is. It fills in blanks that he's just discovered in these past few episodes, but it fills in details that go right back to when he was a child and how he ended up in the United States. It is very specific."

Last week, Hetty (Linda Hunt) returned to the operations center after having been shot by the head of a Romanian crime family with connections to Callen's past. She brought Callen a gift: his mother's name. In fact, Tuesday's episode reveals that Hetty actually knew Callen's mom personally.

That begs the question: Will Callen be able to forgive Hetty for keeping this information from him for so long? "This goes right to the very core of [Callen and Hetty's] relationship," Brennan says. "It creates a very important turning point in Callen's life."

NCIS: Los Angeles: Can Deeks handle a partner swap?

"Does it mean we'll learn everything about Callen? No," Brennan continues. Once you see the episode, you'll realize there's another side of him. He's still ultimately pursuing his identity... and we've still got more to deliver."

Check out photos of the NCIS: LA cast

In the meantime, though, Brennan suggests that Callen — and his team — will be more stable than it has been during his recent quest for the truth. "You'll find that Callen is a little bit more at peace with himself because of what he's found out," Brennan teases. "And the team is just very pleased to have him back to where he was. This is the Callen that they've worked with.  He has resolved some of the issues that were causing him problems, and everything's back on track."

NCIS: Los Angeles airs Tuesdays at 9/8c on CBS.