Directed by German filmmaker Oliver Hirschbiegel (Das Experiment, Downfall) Diana has been described as the "love story between a princess locked in a tower and an ordinary man," the man in this case being heart surgeon Dr. Hasnat Kahn, that lasted from 1995 until a few months before her death in 1997, with her divorce from Prince Charles coming in the middle. So more of an affair than a love story then.
Update: Here's the official synopsis:
Diana takes audiences into the private realm of one the world's most iconic and inescapably public women -- the Princess of Wales, Diana (two-time Oscar® nominee Naomi Watts) -- in the last two years of her meteoric life. On the occasion of the 16th anniversary of her sudden death, acclaimed director Oliver Hirschbiegel (the Oscar®-nominated Downfall) explores Diana's final rite of passage: a secret love affair with Pakistani heart surgeon Hasnat Khan (Naveen Andrews, Lost, The English Patient), the human complications of which reveal the Princess's climactic days in a compelling new light.Hirschbiegel directs from a screenplay by award-winning playwright Stephen Jeffreys, inspired by the book Diana: Her Last Love by Kate Snell, which was in turn drawn from extensive interviews with close friends and confidantes. The result is a window into the tumultuous, change-filled period from 1995-1997, in the wake of Diana's shattering divorce from Prince Charles, and at the moment when she stood on the cusp of adifferent life, evolving into a global humanitarian, a master of maneuvering fame and becoming her own woman.
Diana will be released in the UK in September.