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A Naval Reserve lieutenant, who's also a chemical engineer, dies when a city bus hits her, apparently by accident, on a downtown street in Washington, DC; Gibbs and company investigate; witnesses describe her unusual behavior just before the incident; Palmer notices something weird. The victim's mother tells Gibbs about her atypical background; Abby feels deeply impressed, and she starts to unravel the mystery; she finds a code within a code. Tony and Ziva find a message, then Abby works more on the writings, and she asks McGee to work with her. Duck persists, and he finds a small clue, which leads Gibbs and Tony to a suspect, who collapses before them. The vic's mother gives advice to Abby, then Abby makes more progress. Eventually the gang figure out everything, and they nab the person responsible.