Emma Mastin is a young American housewife who is actually a "Black Widow", a wife who's trained to become a suicide bomber after her husband dies.
Emma's husband, Vakar was wounded in a raid in 2001 by Russian forces and barely managed to survive although upon surviving, he faked his death and went into hiding.
Emma, on the other hand, carrying Vakar's unborn baby and realizing she no longer wanted to be associated with her past, last switched sides and emigrated to the U.S where she gave birth to her baby, a young boy whom she later named Joshua.
Unfortunately, her past life hadn't gone away as in 2010, Special Agent Dan Williams was murdered while on a mission in Cyprus which had the NCIS: Office of Special Projects getting involved. They discovered the truth about Emma's past and with her help, were able to stop or kill the assassins who had intended to kill Emma and claim Joshua.
Emma presumably later came clean to her husband, Clay about her past life and the two along with Joshua were relocated elsewhere although Vakar later made another attempt to get their locations by attempting to capture Kensi Blye and interrogate her for information.
Kensi's partner, Marty Deeks shot and killed Vakar while Kensi herself took care of Vakar's two thugs, finally ending the saga once and for all.