Ducky rocked a 'stache to fool the infamous La Grenouille.
Gibbs called Ducky a "natural" when he went adopted the identity of Charles Harrow and arranged for a high-profile meeting with a world-renowned arms dealer, René Benoit, a.k.a. La Grenouille.
Equipped with a fake mustache and ARES, a powerful and confidential weapons system, Ducky gained direct access to La Grenouille and his cohorts while also snatching a bag of diamonds.
Watch Ducky show off his undercover chops in "Blowback" (Episode 14, Season 4 of NCIS).