On a freeway during a morning commute a Marine staff sergeant, Marcos, dies in an explosion of an SUV; Callen and company investigate; Marcos was a combat engineer who specialized in demolition. TV-news footage reveals nearby a suspicious car, which leads to Winslow, a person of interest. Hunter makes changes; G speaks with her. Sam and Deeks encounter a runner and a bomb; the man talks, but Nell and the guys catch him in his lies, then he tells about four bombs and one target; G and Kensi check out an empty pad, and Sam uses duress to negotiate a deal; Winslow hires a professional to fix his devices; G has another chat with Hunter. A good plan takes an unexpected left turn, but a remorseful man speaks up; then three bombs explode, yet all the gang escape. Nell abruptly discovers Winslow's missing girlfriend, who disappears, as does Hunter. Hetty returns to her office, where she invites G to sit down, then she starts to tell G what she knows about his background.