On March 4, the Martin Luther King Memorial Library in Washington D.C.—an historic landmark designed by Mies van der Rohe that opened in 1972 and serves as the city’s central library—will close for a three-year, $208-million modernization. Dutch firm Mecanoo architecten, working with D.C.-based Martinez+Johnson Architecture, will adapt van der Rohe’s building into a “library of the future.” Included in the plan are improved reading rooms and employee experiences, expanded kids areas, and a new green roof that will serve as a “pocket park.”

The project is an extensive overhaul of one of the most historically important civic buildings in Washington. And a 62-minute film made by Mecanoo, A Legacy of Mies and King, premiering at the library today, documents its evolution and the firm’s sensitivity to the library’s architectural and cultural significance.

There is a narrator, but our real guide is Francine Houben, Mecanoo’s founder, creative director, and the lead architect on the library project, who with her team explores and learns the library, examining what works, what doesn’t, and why. But more interesting is the insight into Houben’s investigation and interrogation of the dual legacies bound up in the building.

As an architect, Houben naturally wants to hew as closely as possible to van der Rohe’s original intentions. Sometimes that means intuiting what he might have done today, based on prior work and remarks. But she’s also an outsider—to both D.C. and the U.S.—who must grapple with her less confident grasp of King’s profound significance. Houben treats both tasks as crucial to solving the riddle of rehabilitating the library. “I constantly have this feeling that Mies van der Rohe is watching from one shoulder and Martin Luther King from the other,” she says.

The film mirrors Houben’s exploration in micro-profiles of both men. With van der Rohe, it presents a sure-footed rundown of his biography, more or less to the point of winning the library commission in 1965. This includes a touching interview with his project architect Jack Bowman, who provides incisive and personal reflections. But the broad-stroke treatment of King’s life is a bit shakier. The film isn’t glib, but rocketing from the Montgomery Bus Boycott to the March on Washington to his assassination feels inadequate. Still, it aims for thoroughness and we’re given invaluable time with Charles Cassell, a civil rights activist who discusses his vital role in getting King’s name on the library.

Ultimately, it’s difficult—even unfair—to judge this film using normal criteria. A Legacy of Mies and King is meant to showcase the work, effort, enthusiasm, and care Mecanoo has put into their plans for the library. But it also has specific audiences in mind: the people who use the library, the broader Washington community, library staff, local politicians and commissioners, and, indeed, future clients. So while viewers may want to jump into a time machine and immediately experience the upgraded building, they are just tourists. Local viewers who actually depend on the facility will—and should—watch more skeptically. Their library is about to close until 2020, its core functions relocated more than a mile away, and this is a tool to ease those pains. But that’s precisely why Washingtonians should see the film. It’s not only useful in appreciating the forces at work, it provides vital context to drive vigilant and informed engagement in what comes next.

Mecanoo architecten’s film A Legacy of Mies and King screens at the Martin Luther King Memorial Library on March 1 at 6:30 p.m. A second screening is scheduled for April 13 at the National Building Museum in Washington, D.C.