The framework is provided by Berge’s journey between each of the pair's opulent homes - from Paris to Normandy to the Moroccan retreat where YSL's ashes were scattered - ahead of a record-breaking Sotheby's auction where many of the couple's possessions - including revered works of art by Mondrian, Goya and other masters - were sold off to eager collectors.
The camera lingers over each of these homes, as though Berge is trying to commit it all to memory. But, as he reveals, selling off a lifetime of possessions is one thing. Saying goodbye to a partner, closing his eyes for the final time and wishing him farewell, is another thing entirely.
Through Berge's own eyes, we see Yves in changing incarnation - from the shy man anointed Christian Dior's creative heir at the precocious age of 21 - to the monikered icon he became in later life. From the beginning, he was confident only in his unique talent for putting together a woman's look and, in his words, making her "confident, assured".
Away from the cutting room, Saint Laurent was a fragile soul, who needed Berge to buffer him from the world beyond, and their enduring partnership demonstrably did much to accelerate French society’s tolerance of gay couples.
There is some great home reel footage of YSL hanging out with Mick Jagger and Andy Warhol - a triumvirate that pretty much summed up the time. Later, Berge's laudation of "that great man" Mitterand is vaguely questionable, but reminds us that these two enjoyed a position central to France's elite. When YSL created the first pret-a-porter line, French living deity Catherine Deneuve was his first customer.
The film is not completely hagiographical. Berge remembers, too, Saint Laurent's inability to deal with his increasing celebrity in the 1970s, when the latter described "fame as the mourning of happiness". In the early days, the designer's mental instability propelled him into an asylum and out of the House of Dior. Later on, Berge was so disenchanted by his partner's reliance on drink, drugs and revelry that he left him, although, as he sweetly admits, he moved only to the end of the street.
This documentary must have been easy in the compilation. It doesn't feel as if YSL lived a day of his life without being photographed, and his distinctive face and style light up the screen. This, and his unique talent, make it easy to empathise with Berge's reflection on his role in the partnership, "I knew my place. We never crossed into each other's sphere."
Berge is in a unique position to reflect on the role of such a creative talent in today's culture, and he moots that "such people reflect our society back at us. We need them to make sense of what is around us".
It would have been nice to have a bit more of this circumspection in favour of another shot of their big house, but this is a sweet love letter of a film from one man to another, while he says goodbye to all they shared.
L'Amour Fou is on release on Monday 7 November. It will be out on DVD on 21 November.