The 13-episode series, which debuts June 24 on CBS, is based on a bestselling Stephen King novel by the same name and produced by Steven Spielberg's Amblin Entertainment and CBS Studios.

"Under the Dome" takes place in a small American town that is suddenly and inexplicably sealed off from the rest of the world by a gigantic transparent dome. Trapped inside the bubble, residents must grapple with post-apocalyptic conditions.

Broadcast networks long ago abandoned ambitious original scripted shows to launch in the summer. Instead, they have held back their promising projects for the fall to kick off the traditional TV season when more viewers are plopped on their couches.

However, by stocking summer schedules with cheaper reality shows and reruns of scripted shows, ABC, CBS, NBC and Fox have witnessed an exodus of viewers, ceding fertile ground to cable channels AMC, FX, Showtime and others. Cable networks took advantage of the broadcast networks' summer doldrums and captured viewers with their own groundbreaking and award-winning programs.

That's why CBS wanted to take a big swing.

"We were looking for the ability to put on more original programming because things were getting a little quiet during the summer, and we had to look for new models to do that," CBS Corp. Chief Executive Leslie Moonves told Wall Street analysts last month.

To pay off on its gamble, the nation's No. 1 network decided early on that it needed to tap two increasingly important revenue sources -- Internet streaming services and international syndication sales -- to finance "Under the Dome" and make it profitable from the start.

The program, which was shot in North Carolina, cost about $3 million to $3.4 million an episode to produce, on the high side for a new network drama. Advertising rates for summer shows were not high enough to support such an elaborate project.

"This series presented us with great opportunities and great challenges," Armando Nunez, chief executive of the CBS Global Distribution Group, said in an interview.

So in February, the network sold the digital rights to Amazon.com, which will stream episodes to subscribers of its Prime Instant Video service a few days after they air on CBS.

Also crucial to the show's financial success was generating interest among international buyers. CBS' mission to sell the show overseas was complicated by the series' unorthodox June launch, and by the fact that there were no full episodes to show to foreign network executives.

"We didn't even have a pilot to show the buyers," Nunez said. But after CBS unveiled a short promotional teaser for the show during the Super Bowl, buzz began building in the U.S. among Stephen King fans -- piquing foreign executives' interest.

Then, last month, the network finally received episodes that could be showcased at the L.A. Screenings, an annual festival that attracts about 1500 international TV buyers to Los Angeles to watch the networks' fall pilots. CBS also arranged screenings in London, Germany and Japan to drum up more interest.

Still in post-production, the episodes lacked much of the sizzle and special effects. That apparently didn't matter.

"When the buyers finally saw actual episodes, we were up and running," Nunez said. "You take a successful network like CBS and the creative auspices that we have for this show, and it just raises the profile of the project and the possibilities for success."

CBS said Thursday that it had sold "Under the Dome" in key international markets, including Britain, Germany, France, Italy, India, Australia and Canada. The program is expected to ultimately run in 200 foreign markets.