Sure, Game of Thrones is popular.But when it comes to the The World's Most-Watched Show, that title belongs to a series that doesn't get nearly as much ink as the question of whether Jon Snow is actually dead. That show: CBS's NCIS.For the last six seasons, the crime drama has more than held its own with cable TV beasts like Thrones and The Walking Dead, with its last season averaging 21.19 million viewers per new episode in the U.S. after 7 days of DVR playback. (Both cable shows hover around the 20-22 million range as well, streams and DVR playback included.)But worldwide, there's not really anything else like NCIS.The show garnered 55 million viewers across the globe in 2014, according to the network, and is licensed in over 200 markets worldwide and in over 60 languages — from Arabic to Vietnamese.And earlier this year, data company Eurodata TV Worldwide dubbed the show "The Most Watched Drama In the World" for the second year in a row. Other shows that have earned that distinction include House, CSI and The Mentalist.But the road to worldwide renown hasn't been a short one.When NCIS premiered in 2003, it was ranked No. 15 across television, with 12.3 million viewers, according to CBS. Strong, but not exactly the top of the TV food chain.It was international audiences, though, that first took notice of the D.C.-set drama and its band of crime fighters, according to David Stapf, President of CBS Television Studios."It was more successful internationally than it was domestically at first," he says. "After the first couple of years, our head of international Armando Nuñez called me and said 'Hey, are you noticing what's going on with NCIS overseas?The series was topping international markets all over the place, he said, like Italy and German.And its overseas appeal was made even more baffling by the fact that the show was about, essentially, investigators within a very specific part of the U.S. military."You would think that would alienate foreign audiences," Stapf says. "But when you look at the show, it's not so much about that. It's about heroes."That's a theme that resonates universally, he says."There's good guys and bad guys and the good guys are going to get the bad guys and they always win. And it's done with [characters] who genuinely love each other," he says. "We all have a Gibbs in our lives." (Mark Harmon has played Naval Criminal Investigative Service leader Leroy Jethro Gibbs for 12 seasons.)"I think [as] time has gone on, people have fallen more and more in love with the characters," Stapf adds. "It resonates all over the world."