Greta Boyen was a neighbor to the Shields family and was married to Greg with the two of them later having an unnamed son.
At some point, while Michaela Shields was overseas, Greta, presumably seeking comfort and bored with her life began having an affair with Shields's husband, David.
David later opted to end the affair much to Greta's dismay. Fueled by revenge and believing that David would come back to her once the Commander was dead, Greta began creating a bomb which she planted in an unmarked car, hoping that once it went off, it would result in the Commander's death.
Unfortunately, Greta's plans failed and NCIS as well as the FBI were all led to believe that the terrorists after the Commander had struck again, not realizing that Greta herself was responsible.
Luckily, Forensic Scientist Abigail Sciuto, having later received copies of the second bombing was able to confirm that the bomb had no connection of any sort to the terrorists. She then called Leroy Jethro Gibbs, informing him of her findings while also causing Gibbs to realize that someone else wanted the Commander dead.
Furious her plan hadn't worked and unable to accept the fact that David had ended their affair, Greta arrived at the Shields house, intending to tell Micki of what had happened, only to discover that Micki had been standing there all along.
Driven to the brink and unable to accept that David would never be with her, a distraught Greta pulled out a gun, intending to kill Micki. David tried to talk her out of it but Greta refused, stating that their lives would be much easier once Micki was gone.
Luckily, Micki drew her gun, resulting in a stand-off between the two former friends, now bitter enemies and without a pause, shot Greta in self-defense just as the other woman was about to fire at her.