The morning-after reaction: Let’s do it again!

Every summer has its surprise hit, and “The Hangover” is starting to look like this season’s unexpected breakout.

Even though the bachelor-party-gone-bad comedy doesn’t open until June 5, “The Hangover” already is generating such positive reactions that Warner Bros. is developing a sequel, a strong vote of confidence for a movie with no big stars, no comic book tie-in and no obvious franchise traits.

Just as the R-rated comedies “American Pie,” “Knocked Up” and “The 40-Year-Old Virgin” transformed excellent word-of-mouth into strong summer ticket sales, “The Hangover” should benefit from the kind of positive moviegoer chatter that largely has been missing from the summer spell -- save “Star Trek.”

If “The Hangover’s” early momentum continues, the movie could challenge Will Ferrell’s time-warp comedy “Land of the Lost” for the top spot at the box office next weekend and corner the adult comedy market until Universal releases Sacha Baron Cohen’s “Bruno” on July 10.

Directed by “Old School’s” Todd Phillips (who parted ways with Cohen during the making of the comedian’s “Borat”), “The Hangover” follows a traditional road trip formula.

Four guys (Bradley Cooper, Ed Helms, Justin Bartha and Zach Galifianakis) head to Las Vegas for a night of typical male debauchery, but when they awake in their trashed hotel suite the next morning, the groom (Bartha) has gone missing. To find him, his three friends must retrace their drugged-out steps from the night before, a strange journey that includes a quickie wedding, a naked Asian gangster, an infant found in a closet, a stolen police car and boxer Mike Tyson’s pet tiger.

While the movie overflows with expletives, it is far less raunchy than some R-rated comedies, and has at its center an unusually good-natured group of friends.

“It’s surprising how sweet a movie it is, which is why it plays so broadly,” says Jeff Robinov, president of the Warner Bros. Pictures Group. “Todd really wanted the movie to have real playability.”

Soon after the studio held its first “Hangover” research screening earlier this year, Warner Bros. decided to start moving forward with plans for a sequel. Contrary to reports in the trade newspaper Variety, the studio has not yet green-lit the sequel nor made deals with its cast, Robinov says. But he said he was hopeful that if “The Hangover” did perform well and all the necessary contracts could be closed, a follow-up film could go into production early next year. “We want to try to get a bit of a jump start on it,” Robinov says.

The studio is equally focused on building up the film’s buzz.

Starting on May 18, the studio launched an extensive screening program, taking “The Hangover” to 80 cities, showing the film at colleges, military bases and radio station-sponsored promotional screenings. In the last week, the film’s cast traveled to eight cities to drum up interest, including Denver, Atlanta, Phoenix, Philadelphia and Miami.

“Everyone who sees the movie loves it,” says Sue Kroll, the studio’s president of worldwide marketing. “It always ends up with people laughing hysterically and everyone wants to go back and immediately see it again.”

Part of “The Hangover’s” challenge will be to attract as many female ticket-buyers as males. The ratio for films like this tends to run 60-40 in favor of the Y chromosome, but Warner Bros. is hopeful that the film’s playful tone will balance the sometimes lewd sexual pranks and ultimately will lead to a 50-50 split.

The studio also must adapt to the personal tragedy suffered by Tyson, who canceled his publicity appearances for the film following Tuesday’s death of his 4-year-old daughter. The studio said it was too late to alter the film’s trailers and commercials, some of which feature Tyson prominently.

Phillips said in an e-mail that he wasn’t sure the film was working until he showed it to 300 strangers and watched them all laugh.

“Honestly, I feel like it’s always a good time for a comedy that works -- a comedy that plays to audiences across the board,” he said. “We’ve screened the film enough times to say with confidence that it plays really well. Whether that translates into a box-office success remains to be seen.”

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john.horn@latimes.com