Early in “Ultrasuede: In Search of Halston,” Liza Minnelli, dressed in Halston stretch velvet leggings (“wonderful!”), gives the director, Whitney Sudler-Smith, a great piece of advice: “Go do some research.”

Among Mr. Sudler-Smith’s first stops? His mom, for insight into what drew him to a topic like Halston. She doesn’t know, and, dear viewer, you won’t either after watching Mr. Sudler-Smith’s bumbling documentary. A bad reporter and an indifferent interviewer, he demonstrates little interest in fashion and less insight into Halston, whose talent and far-reaching influence come as a surprise to him.

It’s not Halston’s clothes — or how they were made or why they appealed to women — that attracts Mr. Sudler-Smith. Instead he’s curious about the girls and the glamour surrounding Halston: Studio 54 and decadent 1970s New York. So he seeks out (among others) Billy Joel, who once used Halston in a song lyric and who compares New York at that time to Hitler’s bunker.

Mr. Sudler-Smith changes his hairstyle and clothes from scene to scene, and he generally finds ways to distract from the topic at hand. Why else keep in bits like his cellphone going off during an interview with André Leon Talley of Vogue? His ringtone: “Dixie.” Mr. Talley’s advice: “Don’t interrupt.”