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On state-owned property near Imperial Beach, California, south of San Diego, while four Marines patrol the area (along the US-Mexico border) to support federal customs and immigration agents, unknown people for unknown reasons ambush the Marines and their Humvee; one escapes, and three remain missing; officials wonder whether the intruders are Mexican smugglers; Callen and company investigate. A search party finds the body of one Marine; the lone survivor says that one of the missing is a pregnant lance corporal, and the dead one was the father; a snitch gives a tip to G and Sam, the latter of whom checks it out while playing it cool; Nate suggests an alternate possibility; while Kensi and Deeks take a bike ride, they find something but run into trouble; G and Sam get a lift and lend a hand, then they find evidence confirming Nate's idea. G and Sam put it all together, so they confront the bad guy in charge. Nate prepares for a journey.