Excellent, well-done episode. It is a good example of why the earlier seasons are better than the recent ones. Besides a good plot, it didn't stop with that but added good extras like character development, humor, good acting and delightful little details and dialog. The quips, surprises and clues flew by rapidly. There were a number of cliche-type things that could have been botched, but were not.
For example, references to Gibbs and the Directors' past romance could have been trite, but avoided much of that. I liked the way Jenny said "don't treat me like a junior agent," and Gibbs replied, "I'm treating you like a junior Director!"
I doubt that anyone got a letter that his sperm bank contribution of 15 or so years ago was unwanted, but it was funny, not overdone, and Ziva's acting was letter perfect.
Ducky's conversation with Abby about her growing interest in autopsies was cute.
A nice story touch was that no one could know if McGee fired the fatal shot, so no completely happy ending.
The plot was good. Some of you might have missed some stuff about the bullets, so I'll quickly go through it here. ***Spoilers.*** There was a gunshot flash near the cop, and McGee --thinking he was fired on-- fired 3 shots at the cop, killing him. The cop was found with 3 bullets (two damaged) in him, so they naturally all thought they were from McGee's gun. The big problems were (1) they couldn't find the cop's gun anywhere, and (2) he was supposed to have fired at McGee (and missed, of course), but they couldn't find any bullets and bullet holes anywhere near where McGee was standing. As usual in many NCIS episodes, Abby, given enough time, solved the case -- she noticed gunpowder on the cop, indicating that at least one of the 3 bullets in the cop didn't come from McGee, rather from someone near the cop. So if McGee fired 3 bullets, one or more had to have missed the cop, and, surely enough, one bullet was found in the wall near where the cop died. Abby also deduced that the dying cop spun around and flipped his gun into the car, thus explaining its absence at the crime scene.