Stephen King made a frightening proposal with Gwendy's Button Box: Write a story with him.

One should never take candy from strangers. Especially if it comes in a container that may have apocalyptic powers.

In Gwendy’s Button Box, the new novella from authors Stephen King and Richard Chizmar, that’s the premise: A heavy-set young girl meets a strange man in a black bowler hat who has a gift for her. She doesn’t know what to do with the buttons and levers on his fancy, peculiar wooden box, but the candy it distributes brings her happiness far beyond a brief sugar rush. It makes her stronger. More confident. More beautiful and brave.

But the buttons … those have a much darker purpose. This is a gift fraught with both responsibility and menace.

That’s how Chizmar, the publisher of Cemetery Dance magazine and a renowned horror author in his own right, felt when King invited him to collaborate on the story, which hits store shelves Tuesday.

“We were emailing in early January and he mentioned offhandedly a story he’d started some time over the summer, and he just couldn’t finish it,” Chizmar tells EW. “As I often do, if he mentions a new story, I always say, ‘Well hey, send it to me if you want. I’d love to read it.’ I didn’t expect anything to come of it. But the next evening a file titled Gwendy showed up, and the body of the email read: ‘Do whatever you want with it.’”

When Gwendy Petersen is given her box, she gets basically the same guidance. “I won’t give too much away, but the buttons promise darkness of the ultimate sort, and the two levers spit out treats that change your life for the better,” Chizmar says.

Unlike some authors whose bylines become brand names, King doesn’t pump out volumes with the help of glorified ghost writers. He previously collaborated on The Talisman and its sequel Black House with Peter Straub, and co-wrote the Red Sox love letter Faithful with novelist Stewart O’Nan. But otherwise, his books are his books. Another exception: he and his son Owen King (author of Double Feature and Intro to Alien Invasion) have co-written the novel Sleeping Beauties, which comes out in September.

So King’s offer to turn Gwendy into a team effort was an extremely rare one.

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RETURN TO CASTLE ROCK

After King’s email arrived, Chizmar immediately read King’s unfinished manuscript of about 7,000 words. “Before I could chicken out, I wrote him back and said, ‘I’d love a crack at finishing this.’ And we rolled with it.”

One of the most daunting things about the story was its setting: Castle Rock, Maine. That small town has deep resonance for any King fan, since it’s the setting for many of his early classics: The Dead Zone, Cujo, The Body (which became the film Stand By Me), and The Dark Half, along with numerous short stories.

King put a tombstone over the community in 1990 with Needful Things, a novel about a charismatic demagogue who turns the townsfolk against each other with promises of bygone glory and carefully stoked resentments: Make Castle Rock Great Again.

Since that conflagration, the town has reappeared only briefly and sporadically in his work, but Gwendy’s Button Box is set entirely within its borders – except for when the box has powers that reach far beyond.

Chizmar admits he was daunted by this return to classic King country. “I felt a huge obligation to trying to do the place and the story justice. It’s sacred ground for Stephen King readers,” he says.

Chizmar felt a bit like he was trespassing, even though he has known the author for years and covered his work extensively through his Stephen King Revisted project, re-reading and reviewing his entire bibliography.

“I spent a weekend in a state of mild terror because I knew what I had done. Then I sat down Monday, and when I started writing some notes it was sheer terror,” Chizmar says. “My hands were shaking. Then I tossed my notebook aside, and opened my laptop and just started writing. Before you knew it, I was in Castle Rock and the nerves were gone.”

ONE STEP AT A TIME

Within three days, he added 10,000 words. “I sent those to Steve. He added a few thousand, sent it back, and then we did that until we were finished. Played ping-pong with emails,” Chizmar says. “Neither one of us ever told the other where we wanted it to go or where we thought it might go. We left that up to the other writer.”

King wasn’t available for an interview, and Chizmar says he’s still not sure what the blocking point was for him. “Honestly, I didn’t ask. Partially, I didn’t want to dip too much in his business and say, ‘Hey, hero of mine, best-selling author of all time, why couldn’t you finish this?’” he says. “I was just so stunned at the prospect of doing this, I didn’t want to go backwards at all. I didn’t want him to change his mind.”

They spent exactly a month on the novella. King sent him the email on Jan. 6, and he sent his final revisions on Feb. 6 of this year. “It’s a huge amount of trust, especially for him. It was really easy for me to trust Stephen King with my words,” Chizmar says. “On the other hand it was pretty amazing and wonderful to see the respect he gave to the words I put on paper.”

In addition to Castle Rock, there’s another iconic King presence in Gwendy’s Button Box. The mysterious man in the bowler hat, who watches the young girl as she sprints each day up the winding metal “suicide stairs” of Castle View Park, is named Richard Farris.

That won’t mean much to newcomers, but the initials RF usually portend someone closer to the anti-Christ than the Good Humor Man. Is this another manifestation of Randall Flagg, the demonic figure from The Stand and The Dark Tower?

Chizmar offers only a hesitant, “Maayyybee,” before adding: “He’s definitely mysterious, and it’s really obvious there’s more to him than meets the eye.”

THE GIFT THAT KEEPS ON TAKING

What he offers to Gwendy is not so much a gift, but a test. The tiny chocolate treats the box dispenses are addicting, and they begin reshaping her – literally – for the better. But with that comes a temptation to explore the box’s other powers.

“Considering the world we live in right now, and our leadership, I think Gwendy packs a lot of emotional layers to it, and each character is a microcosm of where we are today and what we’re capable of, both good and bad,” Chizmar says.

All Gwendy wants is to stop people from making fun of her, but that’s also a path to darkness. When people get hurt, they want revenge. But is that who she really is?

Has the man in the bowler hat miscalculated – or is she exactly the custodian he needs, one who will be as careful with it as Chizmar was with King’s seed of an idea?

“I think she’s a lovely character, with a great heart, who does a much better job with that box than I think most of us would’ve,” Chizmar says. “Myself included.”