HBO announced Tuesday it will air a “landmark documentary” about the late Princess Diana, in line with the 20th anniversary of her death.

The documentary, which the network has labeled “unprecedented,” will focus mainly on interviews with the princess’ sons, William, Duke of Cambridge, and Prince Harry, who open up about their mother and how she influenced them.

Included in the documentary will be new archival materials along with reflections from friends and family, some of whom have never spoken publicly. Will and Harry will also discuss Princess Di’s philanthropic endeavors and humanitarian work.

“This film will show Princess Diana in a way she has never been seen before, through the eyes of the two people who knew her best,” executive producer Nick Kent said in a statement.

HBO will air the documentary ― produced by Oxford Film and Television ― in the U.S., while ITV will air it in the U.K. It’s set to air later this year, although no specific date was noted.

In light of the anniversary, HBO isn’t the only network with an upcoming Princess Diana documentary.

Back in February, ABC announced its plans to air a docu-series focusing on the late royal, which is set to air this summer. Princess Diana and Prince Charles will also serve as the subject of the second season of Ryan Murphy’s “Feud” anthology series on FX, set to air sometime in 2018.