Customers who walk into the Tropes & Trifles store know they're about to encounter a happy ending. In fact, it's guaranteed.

Tropes & Trifles, which opens Saturday, is a romance-only bookstore and romance books have two unbreakable rules: A love story must be front and center and the book must end happily.

"This is the one section where we can't guarantee you a happy ending or, at least a happy ending for the romance," said Tropes co-owner Caitlin O'Neil, pointing to the small "Not Quite Romance" shelf, which contains, for instance, thrillers with love stories that aren't the focus of the plot. All of the other books in the store — roughly 5,000 of them — promise a wedding or at least an ardent kiss at the end.

With romance books accounting for almost 20% of U.S. book sales, the south Minneapolis store may be on the crest of a boom, with love-themed bookshops shaping up as a trend.

"We thought we would be the fourth [all-romance store] in the country. Then, five opened last year," said co-owner Lauren Richards, who said the nearest stores she knows of are in Chicago and Kansas. Tropes had been doing pop-ups at breweries and themed events, selling up to 100 titles in a weekend, but the two decided last fall to open a brick-and-mortar store, at 2709 E. 38th St. (across the street from Northbound Brewpub and a couple of blocks west of the 38th Street light rail station).

The intervening months have been filled with construction that expanded the sales space from 500 to 800 square feet, ordering books and shelving books, in categories that include historical, paranormal, contemporary and dark romance (said O'Neil, "This is where you have people falling in love with villains or morally gray characters."). There's also been a lot of paper-folding — above the Romantasy shelves is a wall of colorful origami dragons, which Richards folded while bingeing "Love Is Blind" episodes.

The woman met five years ago in a book club. A romance book club. That's where Richards, who had been a political consultant, first joked about opening a bookstore. A month later, she joked about it again.

"I was like, 'Twice is a pattern,' " said O'Neil, a book publicist and translator who direct-messaged Richards to find out how serious she was.

"I said, 'I have a budget spreadsheet,' " Richards recalled answering, to which O'Neil added, "And it was a beautiful budget spreadsheet."

One reason the romance genre has become so popular is the recently amped-up diversity of authors and stories, such as the "Bridgerton" series and "Can't Escape Love," whose heroine uses a wheelchair.

"It's our philosophy: A love story is a love story, no matter what the people look like," said Richards. "It might have queer people. It might have Black people. It might have straight people. It might have hockey players."

The two also think the pandemic drew more readers to romance.

"There was so much anxiety and uncertainty," said O'Neil, that readers turned to books where they knew mental health concerns and grief would be plot points, but that everything would work out just fine. The happy endings made it easier to deal with. But happy endings do often imply sad beginnings.

"These books are exploring things that are difficult, like grief," said Richards, who jokes that she developed an affinity for reading about sickbeds in romance books because she was confident that they wouldn't turn into deathbeds. "There might be an illness that, in real life, probably would have killed the character but [the books] can explore those things and you know everything is going to be OK."

Another shift is that romance may be paramount in the genre, but it's not the only thing the characters are chasing. Said O'Neil, "Often what happens throughout the telling of the story is they find romantic happiness but also something that fulfills them spiritually or professionally."

That's why the phrase "Spoiler: There's a happy ending," recurs frequently in conversation with Richards and O'Neil.

One happy surprise for the co-owners has been the reception from other independent bookstores. They reached out to many — including neighbors Moon Palace Books, DreamHaven Books and Irreverent Bookworm — whose owners gave them business advice and added them to local group chats.

One of the challenges has been figuring out how to maximize the small Tropes & Trifles footprint. The pair have taken sort of a Murphy bed approach. Many of their shelves are mounted on casters, so they can be moved aside to make room for a reading or book club meeting.

"There's a reason a lot of these things are on wheels," said O'Neil.

She and Richards did a few pop-ups in February, which reminded them that what they've missed most about not selling many books while they oversaw construction was chatting with romance fans and swapping recommendations (both read two or three romance books a week).

"There is such community in romance," said Richards. "People love to read the books, but they also love to talk about them."

They'll be able to do that at the grand opening, from 11 a.m. to 6 p.m. Saturday. The event features signings by Iowa writer Denise Williams ("Technically Yours") and Minnesota writer Laura Moher ("Curves for Days"). Masks will be required from 3-5 p.m. Happy endings will be required always.