Last night's NCIS: Los Angeles opened with something cool we don't get to actually see that much of - Callen going deep cover, presumably long term, under an alias.

That all changed after he was "Burned," however, and the episode soon evolved into a frantic hunt for the person who just masterminded a major NCIS security breach.

For G. Callen, it also triggered a new effort to learn more about his own past.

Naturally, G. picked up that he was being tailed, and turned the tables on the outmatched P.I. But the fact that Callen's cover was blown was extremely problematic.

The guy tailing him was killed by a car trying to escape, but was just a minor player. Eugene Kilson was the real brains digging into G.'s past - and blackmailing him with it.

Callen was asked/forced to exchange a flash drive to a gang of armed Bulgarian thugs in exchange for the truth about himself. He complied, masquerading as Kilson.

His cover blown, Callen did his best to hide in plain sight.

After the drop off went awry, gunfire ensued and Callen escaped ... barely. The dual mysteries of the flash drive and its connection to our agent were starting to intertwine.

Meanwhile, Kilson hacked into NCIS 'mainframe, forcing Eric to take the office offline. Not good. But NCIS was able to discover what was so valuable about the flash drive.

It contained a travel itinerary on a business executive coming from Europe to close a merger that would have sent stock prices tumbling. The gangsters weren't having it.

NCIS intervened to rescue the exec and killed all the thugs but one. Callen arranged for a meeting with Kilson in broad daylight, but ended up killing him in self-defense.

Did the secrets of Callen's past die with him? Not necessarily. The team finds a key to an office with Kilson's files, presumably including info on Callen and much more.

We don't know who hired Kilson to find Callen and expose NCIS, but this story line is only beginning. As the first of a two-part season finale, it will pick up next Tuesday.

This was a typically solid episode, and definitely leaves us wondering how badly the OSP is/was compromised, and by whom. However, a couple of points don't add up.

The OSP is this undercover, high-tech division of NCIS, yet we don't see Callen work undercover quite often enough to understand the dire implications of getting "burned."

On a similar note, Callen often refers to himself as such. How did he immediately recognize how bad the NCIS security breach was when the dying man gasped his name?

Callen’s “mysterious” past in general is also something that seems just a little forced. It's great in that it fits his character, and how he immerses himself in his alter egos.

But does the fact he doesn’t know his birth name really matter? He had a tough upbringing, but what does he really not know - and willingly takes huge risks to find out?

We'll give the show the benefit of the doubt, at least until next week. We just hope it didn't overreach and set us up for either an implausible stretch or a huge letdown.

A couple of other thoughts and observations from the episode:

• Loved the mentions of the agent exposed and killed on both shows, Dom and Lara Macy (see our NCIS review from last night). Subtle tie-ins like this always beat forced special guest star ratings gimmicks (Director Vance excepted).
• Eric being forced to take the office off the grid was like having part of him amputated. You don't often see the world's coolest computer nerd that flustered.
• Sam and Kensi's discussion of "running game" was hilarious.

What did you think of last night's NCIS: Los Angeles?

Steve Marsi is the Managing Editor of TV Fanatic. Follow him on Google+ or email him here.