The strengths and the limitations of taking a personal approach to documentaries can be seen in Hannah Rothschild's The Jazz Baroness, which was shown at the Telluride Film Festival and has been bought for broadcast on HBO.

Twenty-five years ago Rothschild learned that her wealthy great-aunt, the Baroness Pannonica Rothschild, had been a major patron of jazz -- especially of pianist Thelonious Monk -- during the 1950s and '60s. Although she met her aunt only a few times before her death in 1988, Rothschild has spent the last couple of decades ferreting out as much as she could learn about her unorthodox relative. The resulting film is frustratingly incomplete at times, but it highlights a most intriguing figure.

Rothschild narrates the film herself, and Helen Mirren provides the voice of Pannonica in excerpts from the elder Rothschild's interviews and diary entries. We are told a little more than we want to know about Hannah's connection to her great-aunt, and a little less than we want to know about the American jazz scene that Pannonica savored.

Pannonica was a member of the famous Jewish banking family but she was bored with the idle life expected of her. On a visit to New York, she heard Monk play and became inflamed. She immersed herself in the jazz world and ended up in the tabloids when Charlie Parker died of a drug overdose at her residence in 1955. In 1958, Pannonica was arrested while driving Monk to a concert; drugs were found in her car.

Several people who knew them, including Monk's son, suggest that Pannonica was in love with Monk, but since he was married, it's unclear how far their relationship progressed. Probably the feelings were more intense on her side than on his, though the musician wrote several songs for her, so clearly she was an important figure in his life.

Among the people interviewed in the film are several members of the Rothschild family, jazz musicians and aficionados including Quincy Jones, Sonny Rollins, and Clint Eastwood. Several things are left disappointingly murky, including Pannonica's relations with her children after she left her husband to follow Monk around the world. On the whole, however, the evocation of Pannonica is rich and revealing. But one longs for deeper insight into Monk's personal and professional history. This frankly subjective portrait provides a wealth of revealing details, but we can't help feeling there is a much larger story still waiting to be told.

HBO