Mark Harmon and Muse Watson, NCIS

NCIS' Leroy Jethro Gibbs (Mark Harmon) has always been able to turn to his mentor, Mike Franks (Muse Watson), in times of trouble. When Franks returns in the hit CBS procedural's Tuesday episode, the tables will be turned.

Two mercenaries are found dead on Gibbs' boat, which forces Gibbs to re-evaluate his old friend and talk his way out of some hot water.

Check out photos of the cast of NCIS

"We find out where the boat has ended up and it's something that Franks is involved with," Watson tells TVGuide.com "Franks and Gibbs have been such dear friends, so Gibbs lets Franks have the boat and everything gets confused from there. And of course when Vance finds out that Gibbs' boat is involved, they want answers from him and he doesn't have any."

The episode, which also features guest stars Robert Patrick (Terminator 2: Judgment Day, The Unit) and Diane Venora (Romeo + Juliet, Bird), also forces Gibbs to play family counselor for Franks. Watson (Prison Break) says it's Gibbs' turn to return the favor to Franks, who helped Gibbs deal with the death of his wife and daughter.

"You'll find out a little more about Franks' family, and I think the audience will be delightfully surprised at the way Franks has changed as far as his relationships with his relatives are concerned," Watson says. "I think it's drilled into the audience that family is the most important thing in the world. What made Franks help Gibbs with his wife and daughter being killed is what gives Gibbs a little more patience with Franks in this episode."

Watson, 61, also points to Franks assistance with that issue as the most important turning point in their relationship. He says he enjoys seeing Gibbs display traits he's learned from his mentor.

NCIS: Robert Wagner to Play Tony's Dad

"When Franks surreptitiously gave him the information about the sniper that killed Gibbs' family, he could act on it or he could go to somebody else and have them act on it," Watson says. "I think the fact that he acted on it endeared him to Franks. They became even closer than they ever were before.

"He and Franks are both willing to go the extra mile to make sure justice is done, even if we can't dot our I's and cross our T's on the paperwork," Watson says. "It's a very special relationship, and I think the roles have reversed somewhat. I think as a mentor, he has come into his own."

Does that mean Franks won't be coming around as much? Watson says he and the writers have discussed his next possible appearance. Though nothing is set, he likes his chances.

"Every time I walk on set, even the crew guys say, 'Oh goodness, Mike Franks is back. This is going to be a good one,'" Watson says. "They always know that something good and something strange and something secretive is going to happen. [They know] the cowboy aspect of Franks is going to come out and he's probably going to be blowing some people away."

NCIS airs Tuesdays at 8/7c on CBS.