NCIS: Los Angeles Season 1 Episode 23 (05/18/10)
The episode opens with Callen being tailed, which he picks up on instantly. A chase ensues, with Callen turning the tables on his stalker.
The man is killed in a car crash trying to escape, but before he does, Callen learns he's been made. He cuts off communication with NCIS.
A man contacts him, though, by the name of Eugene Kilson. He was hired to dig into Callen's past for reasons unknown, and is using that to blackmail G.
Callen has to drop off a flash drive to some Bulgarian thugs or he'll never learn the truth about himself. He complies, but the drop goes awry and gunfire ensues.
Meanwhile, Kilson hacks into the NCIS mainframe, forcing Eric to take the office down.
The flash drive contained a travel itinerary on a business executive coming to down to close a merger that would have sent stock tumbling. The gangsters wanted to put a stop to that.
NCIS intervenes and most of them die, save for the lead thug, who also evaded capture by Callen in their previous encounter.
Callen agrees to meet Kilson, who he identifies as the man who ran over the man tailing him at the very beginning. He chases him down and kills him after Kilson draws on him first, perhaps burying his secrets too.
Or not. The team finds a key to an office with files Kilson kept, presumably including information on Callen and a whole lot else. To be continued ...