What Michael Phelps does in the dark has earned the spotlight for Under Armour and Droga5 at the 2016 Cannes Lions International Festival of Creativity.

An ad from the "Rule Yourself Campaign" and featuring the Olympic Gold Medalist preparing for his last shot at glory took home the Grand Prix in the Film Craft Category, which honors "the quality and aesthetic of the individual elements of the film-making process."

Or, as jury president and Great Guns Founder and CEO Laura Gregory put it: "We're looking for details and choices." That includes, among other things, casting, animation, visual effects, sound design and more. "It's not just about the idea," she said.

The Phelps film was directed by Epoch's Martin de Thurah and documents the swimmer's grueling training regimen both in and out of the pool. Beautifully shot and set to the "The Last Goodbye" by The Kills, it's a reminder that this is likely to be Phelps' last Olympics. It also continued Under Armour's tag line "It's what you do in the dark that puts you in the light."

Ms. Gregory mentioned a great number of pieces featuring diversity and gender, some of which were good and many of which weren't. She also seemed to call for a stop to spots making music out of strange objects like car doors.

"The cream of the work," she said, "was exceptional" and there was a "major debate" over the Grand Prix.

The other contender for the top prize was another spot from Droga5, "The Piccards" for LVMH's Hennessy brand. Creativity described the work as "a sweepingly cinematic spot that … illustrated the brand's metaphor for the relentless pursuit of achievement."

As Ms. Gregory put it, the Hennessy work was "this beautiful piece, but felt like it had come from a movie," while "Phelps" was beautiful and "took you inside the head of someone making a comeback."