The challenge: Drive demand for meatless burgers in Chicago, a city with a meat-packing heritage and dining culture centered around hot dogs, Italian beef and sausage. This is, after all, the city where Ferris Bueller impersonated Abe Froman, the fictional "sausage king of Chicago," on his legendary day off.

Yet that was the task of the Impossible Burger, a veggie burger made from plants that's meant to look and taste like the real thing. Some describe it as "juicy," "sizzling," or even "bleeding," even though it's made without meat. It comes from a Silicon Valley startup on a mission to prove plant-based foods can be just like real meat, or close enough to it. The burger, for now its first and only product, is made with ingredients including textured wheat protein, coconut oil and leghemoglobin (soy). The last one is so unfamiliar that it gets its own explanations on the company's FAQ page.

Impossible Foods was started in 2011 and began selling its burgers to restaurants in 2016. It has the backing of investors including Khosla Ventures, Bill Gates, Google Ventures, Horizons Ventures, UBS, Viking Global Investors, Temasek and Open Philanthropy Project.

Now, it's in a major expansion mode, with a new factory set to make 1 million pounds of its burgers per month to be sold in restaurants in cities such as Boston, Houston, Los Angeles and Houston, though they're still not for sale to the public in retailers. In June, it hired Wieden & Kennedy to help with creative efforts after handling them in-house.

In New York, the company relied on the backing of celebrity chefs including David Chang, who spread the word with the product's July 2016 debut at his Momofuku Nishi restaurant. But Jordan Schenck, director of marketing for Impossible, said it wanted to move to "ownable experiences" for the brand. So it recreated another Chicago tradition: the speakeasy. On an elaborate stunt to promote Impossible Burgers, for two days -- Oct. 24 and 25 -- it turned a restaurant into what it called a "meateasy."

First, it began posting signs from a made-up "Chicago Sausage Guild" urging people to halt "meateasies" -- secret places where people go to indulge in plant-based burgers without invoking the wrath of the meat-boostering guild. A hotline, 1-833-2SAUSAGE, was set up with a Chicago-accented recorded greeting from the Guild and led callers to a guild website. Some of the posters were defaced on purpose with a secret society logo to keep the idea rolling.

"We could have easily just done the secret Meateasy and that would be it, but we had a build up to it," says Jason Campbell, creative director, Wieden & Kennedy, Portland.

For the two-night takeover, the restaurant Black Bull was transformed into a 1920s butcher shop. Signs in front looked real enough that some people stopped in to see if they could really get short ribs, pork chops and other meats for well under $1.

Outside, Impossible had actors dressed as picketers from the Chicago Sausage Guild. Inside, other actors dressed like butchers, bouncers and flappers interacted with about 400 guests over the two evenings, some of whom showed up in suits and bow ties. "We wanted to create something that felt like more than just a theme party," says Kimmy Cunningham, Account Supervisor, Wieden & Kennedy Portland.

A few people in the meateasies were presented with hats with hidden compartments. While such compartments might have stored alcohol during Prohibition, they were meant to hold the Impossible Burger. The burgers and fries came from local restaurant M Burger, which has been serving the patties since September. Schenck declined to say how much Impossible Foods spent on the Chicago event but said even with a bit more "flair," it was in line with its standard launches.

Along with Wieden, partners on the project included AllisonPR, Event Creative on production, and catering by M Burger at Black Bull's venue.