Alex Tanner was a Lieutenant Commander in the United States Navy and also the head of Project Honor, a high-security project.
In addition, he was also the father of Zach Tanner.
Lieutenant Commander Tanner was kidnapped and tortured by mercenaries into giving information in regards to the Project with the mercenaries showing him proof that they had his son.
The three successfully killed the mercenary about to murder the Lieutenant Commander and ultimately rescued Tanner from captivity with the Lieutenant Commander stating that he'd had to go along with the merceranies's demands due to the fact that they had his son.
NCIS later arrested Harry Wilder, a Commander who was later revealed to have been in league with the mercenaries while also reuniting Lieutenant Commander Tanner with his young son.