Charlotte, an "oversexed Barbie doll" who carries on correspondence with dozens of prison inmates in the late 1960s.
One day a letter arrives that stands out from the pile, mainly in its graphic account of the sex that convict Hillary (John Cusack) proposes to have with her when he gets out. Charlotte promptly declares him The One.
Nicole Kidman in Paperboy
Meanwhile crusading newspaperman Ward (Matthew McConaughey) learns of Hillary's case and believes he has been falsely sent to death row for the killing of a policeman.Ward, his brother Jack (Zac Efron) and a black writer Yardley (David Oyelowo) begin to investigate with the help of a dossier compiled by Charlotte, whose tacky sensuality proves irresistibly mesmerising to the fresh-faced Jack.
The group's first visit to Moat County prison turns into a farcical seduction, as the sex-starved Hillary commands Charlotte to simulate a sex act at a distance as the red-faced reporters look on.
The Australian Kidman, who dials down her natural glamour and turns up the sex appeal in the role of a trashy seductress, said she had been looking for "something raw and something dangerous" when she was given Daniels' screenplay.
"I wanted to be in his hands and see what he would do," said Kidman, who was in selection at Cannes in 2003 with Lars Von Trier's Dogville. "I may be uncomfortable watching the movie, but that's my job."
As his characters journey into the moist heat of the Florida swamps, Daniels probes a dank and violent underbelly of American society.
"There's something very erotic about the 'swamp', something very magnetising about that world and these people," McConaughey told reporters.
"Murky, mysterious. Everyone in this film, everything is not as it seems. That's where Leonardo loves to spend his time, in the gaps in between."
Daniels took a novel by Pete Dexter as his starting point, but worked in the theme of race relations through the added characters of the black writer and the housekeeper who narrates the tale.
"What I could give in this world was my truth," the African-American director told the press conference. "Every character in this movie is someone that I know personally," he said.
The young Jack has an affectionate relationship with the maid Anita (the singer Macy Gray) in spite of the fraught race relations of the era.
"I watched a movie called The Help and though I liked it 90 percent of my family was 'help'," Daniels said. "And these are the stories that they told, they loved the people they worked for."
For the prison plotline, Daniels also drew on personal experience, having raised his brother's two children after the latter was jailed for murder.
The film also touches on homophobia, in particular the issue of interracial gay sex.
"I can't tell you how many men I've been with the 80s and 90s, that were white, that I could be intimate with and that would shun me in public," the director told reporters.
The film, one of 22 in the running for the festival's Palme d'Or top prize to be handed out on Sunday, divided the audience at an early press screening, drawing a mix of applause and boos.
Daniels' first feature film, Precious, the story of an African-American incest survivor, screened in the festival's Un Certain Regard sidebar section before going on to two Academy Award nominations.