Long before he opened 80 Thoreau in Concord, Ian Calhoun had his eyes on starting a restaurant in Boston. Now, more than six years after opening that beloved suburban spot, Calhoun is set to fulfill his earlier ambitions with Mooncusser Fish House, slated to open in late July across from the Castle at Park Plaza.

“This fish restaurant idea existed in my mind at the time. I think now is the time to take this kind of opportunity,” says Calhoun, who’s having Concord staff reprise their roles at Mooncusser. “Doing it with Carolyn [Johnson, chef] and Vincent [Vela, maître d’] gives us a lot of experience.”

The three-story, 4,800-square-foot triangular space provided a design challenge for Calhoun, but he settled on splitting it into a casual bar serving lunch and dinner on the first floor and a formal dining room on the third floor, with the kitchen occupying the second floor.

“It’s still a small space, but when you see it without any partitions, it’s pretty cool,” Calhoun says. “We have the same template on both floors, and we designed a fine-dining restaurant within it, and we designed a fun bar within it.”

Dubbed Moon Bar and set to open a few weeks after the dining room, the first floor has a 30-seat triangular bar in the middle of the space and an open kitchen area behind it. Additional seats scattered along the window and in the back of the space bring Moon Bar to 48 seats. The downstairs menu will include simpler fare that’s quicker to prepare such as lobster rolls and fish tacos as well as a beer-and-wine list with an extensive by-the-glass menu of white wines that will tap into Mooncusser’s 400-bottle selection.

The third-floor fine dining area will feature white tablecloths, pine floors, recessed lighting and seating for 60 that includes a series of two-top tables in the middle of the space and four-top tables at a long banquette along the windows. The upstairs menu will include entrees such as a pan-roasted striped bass with beans, cabbage and truffles as well as a stuffed skate with lobster, potatoes and baby carrots. Among the starters will be fluke crudo and smoked bluefish pate, and a five-course tasting menu of local fish will be available every night.

“It’s more elegant and refined upstairs,” Calhoun says. “That’s reflected in all the things that go into the dish. It doesn’t mean there won’t be the same kind of thoughtfulness and prep downstairs, but when it comes to putting dishes together, it’ll be at a different pace up here.”

And Calhoun hopes that Mooncusser can offer something different from the oyster bars that have popped up in spots across the city. “While we’ve all enjoyed the amazing success of the oyster bar family, for us we’re putting fish at the top of the stack,” he says. “It’s not that we won’t serve shellfish, but we’re focusing on fish.”

Mooncusser Fish House 304 Stuart St., Boston (617-917-5193) mooncusserfishhouse.com