Developers on Wednesday filed plans for a 1.7 million-square-foot complex of housing and office space along the Mystic River in Charlestown, a big project that would plug an even bigger hole on the city’s sea-level-threatened waterfront.

The Flatley Co., longtime owners of the Schrafft’s building and neighboring parcels on Charlestown’s old Mystic riverfront, gave formal notice to the Boston Planning & Development Agency of plans to redevelop 425 Medford St., a 20-acre parcel that once housed the Domino Sugar refinery, into a large mixed-use project. Read more here.