This is a tag to any episode when NCIS agents (not just the team)
have been responsible for protecting someone or just 'minding' them.
So that's The Meat Puzzle, Bloodbath, Cover Story, Iceman, Child's Play and Defiance.

GIBBS' NEW RULE.

"Follow me!" The order barked by their boss causes the agents to jump from their seats and hurry after him as he heads to the elevator. As they step into the evidence garage they see one corner has been set up as a classroom with desks, chairs and a whiteboard turned the wrong way.
"What's going on Gibbs?"
"Sit." They do exchanging puzzled glances as they wonder what's going on. "You're going back to school."
"Do we have to?" Sometimes Tony really doesn't know when to quit but the rapidly delivered head slap silences his whine.
"Last night two FBI agents were killed while protecting a witness in a big case."
"Who or should that be whom?"
"Ziva! I hardly think now is the time. Who was it?"
"Agent Sacks and Agent Thomas. The witness was killed as well."
"Sacks? Didn't he just have a kid?"
"Yes. A son." The team is silent for a moment.
"How did it happen?"
"Do you really think how is important McReason?" Tony turns to him. "Dead is dead McGee."
"But Tony I was just..."
"It is very important DiNozzo. That's why you're here."
"O.K. Shutting up now."
"Fortunately there was also video surveillance in place so we have a pretty good idea what happened. Agent Sacks left the unit to get something from his car."
"Oh." Tim goes pale as he remembers what happened when he did that very same thing.
"We have all done it McGee." Ziva tries to reassure him as she remembers when she was watching Angela.
"It was all over by the time he got back. Apparently when Agent Thomas heard a knock at the door he checked through the peephole..."
"Good procedure there."
"You think DiNozzo?"
"Yeah. It's protection detail 101. Always check before opening the door." DiNozzo grins smugly confident that he is right in this.
"Well good procedure got Agent Thomas shot through the eye." The team falls silent again as they contemplate such a death.
"So what are we meant to do?"
"Abby's working on a mirror device to solve the checking the door problem. What I want you to remember is the new rule thirty-two."
"Rule thirty-two. Wasn't that something to do with lawyers?" Tony muses.
"Yes but there's rule thirteen to cover that."
"So what's the new rule?"
Gibbs flicks over the whiteboard to reveal the one sentence written there. Rule 32 - Never, ever leave a protectee. The agents stare at it for a moment before reacting simultaneously.
"Good rule Boss."
"Nice one."
"Like it."
"And don't forget it" he orders glaring at them each in turn before striding off. As the elevator doors shut he can't stop the memories of agents long gone. Losing a protectee would be hard but nowhere near as hard as losing one of the team.