It wasn't all that long ago that the broadcast networks typically went into hibernation after May. The summer months? Mostly a wasteland riddled with reality shows, reruns and rejects.

But last year, CBS altered its game plan with "Under the Dome." It was a high-concept "event" series that came with a hefty price tag and a couple of big names attached: horror maestro Stephen King, whose 2009 novel inspired the series, and Steven Spielberg, one of Hollywood's most famous directors.

The result was a monster hit. "Under the Dome," a wild tale about small-town residents who find themselves mysteriously trapped under a huge, transparent bubble, lured nearly 18 million viewers to its debut episode, becoming the most-watched scripted summer premiere in the United States in 21 years. Moreover, the show, which also aired about the same time in 200 markets across the globe, turned out to be one of the most popular dramas in the world.

How popular? Alexander Koch, who plays Junior Rennie on the series, recalled a visit for a promotional event last year in Mexico City, where he was overwhelmed by fans and media.

"I remember thinking, 'Whoa! This is a lot bigger than I even thought it was,' " he says. "It was so weird. People were going nuts over the show. It was a huge eye-opener."

"Under the Dome," which returns Monday for its sophomore season, is the TV equivalent of a big-screen popcorn blockbuster -- an epic, scary, sometimes kooky, special effects-laden escapist thriller. It's also an example of how the old rules of television are quickly vanishing.

"Now, everybody apparently wants to go big during the summer," says Neal Baer, an executive producer for "Dome" who also wrote and produced for "ER" and "Law & Order: Special Victims Unit." "You're seeing fresh, scripted content in a lot of places, and viewers seem to want it."

Indeed, "Dome" will be joined on CBS this summer by "Extant." Set in a futuristic world, it's another sci-fi saga executive produced under Spielberg's Amblin Television banner -- and it's bolstered by a high-profile draw in Oscar winner Halle Berry.

She plays an astronaut who returns from a 13-month solo space mission, only to discover that she's inexplicably pregnant. And, as it turns out, this pregnancy could change the course of human history. "Extant" is scheduled for a 13-episode run.

During a media session to promote the series, Berry noted that she signed on for her first recurring TV gig because "Extant" is "on par with any film you'll see." She also was eager to work on a Spielberg project.

"I was a big 'E.T.' fan," she said. "That's my version of science fiction, which is why, when I heard Steven was involved, I was really excited, because that's the kind of science fiction I think I really like. It has a lot of heart, but it is sort of supernatural. That's what hooks me."

Meanwhile, other networks are jumping on the summer "event" series bandwagon. Fox notably revived Jack Bauer (Kiefer Sutherland) for a turbocharged 12-episode reboot of "24," filmed in London, and NBC has John Malkovich starring as Blackbeard in the pirate saga "Crossbones."

Over on cable, FX teamed with director Guillermo del Toro on his horror epic "The Strain," which reportedly carried a $9 million price tag just for the pilot episode, which premieres July 13. And earlier this week, TNT debuted "The Last Ship," an action-packed drama about a global catastrophe from producer Michael Bay.

Michael Wright, the programming chief for Turner Networks, recently told media buyers that the goal is to make watching TNT be like going to a great summer movie.

"Grab a bucket of popcorn, kick back and let us take you on a thrilling ride," he said.

Of course, going big carries some risks, and requires, in most cases, creative financing plans. For "Dome," CBS struck a mold-breaking deal with Amazon that would allow Amazon Prime members to exclusively stream episodes of the show only four days after they debuted on the network. According to the Hollywood Reporter, the deal delivered more than $700,000 per episode in licensing fees. In addition, CBS banked on the strong international appeal of the King and Spielberg names to license the show internationally. CBS basically is following the same financing game plan with "Extant."

But television has a challenge not faced by its big-screen counterparts: The excitement -- and ratings -- must be sustained over an extended period. It's no wonder, then, that "Dome" producers are hyping the fact that King himself was brought in to pen the first episode of the second season, and he also makes a cameo appearance.

"We felt that, with his imprimatur and creative vision, he could give us a great start to Season 2," Baer says. "And it wasn't like he was just writing one episode. He had to think about things that would set up plot lines for the entire season."

To that end, King has promised "some very scary, mind-blowing stuff," while Baer says the show will introduce several new characters, including one played by "ER" vet Sherry Stringfield, and explore issues of "science vs. faith." Moreover, the emotional stakes will be raised with the deaths of at least two beloved characters.

"No one is safe under the dome," Baer teased. "As it turns out, that 'Game of Thrones' isn't the only game in town."

Follow Chuck Barney at Twitter.com/chuckbarney and Facebook.com/bayareanewsgroup.chuckbarney.

'under the dome'

When: 10 p.m. June 30
Where: CBS

'Extant'

When: 9 p.m. July 9
Where: CBS