At the core of this intriguing feature documentary is Louis Kahn, an enigmatic and mysterious man, dead for over thirty years, and his twin legacies: Louis I. Kahn the celebrated modernist architect, and Louis Kahn the father and husband. Married with one daughter, he also maintained relationships with two other women during his lifetime and had a child with each. It is one of these children, his son Nathaniel, whose search for the truth is the basis of this film.

Kahn’s unusual death, which took place when Nathaniel was eleven, provides a good departure point. Having suffered a heart attack in the toilets of Pennsylvania station in 1974, Kahn had scratched over the name and address on his passport, and could not at first be identified. Forever leaving unanswered the question of his intentions, Nathaniel thinks – or hopes – that Kahn may have been leaving his wife to come and live with them. Keen for any scraps of evidence, he interviews a wide range of Kahn’s former colleagues, family members and one-time companions, his travels taking him through America and across the globe to the places his father worked.

Architects of note, such as Frank Gehry and I.M. Pei, provide fascinating insight into Kahn’s work and also heartfelt admiration for the man; Edmund Bacon, planner of Philadelphia and one of the film’s more colourful characters, gives an amusing, and refreshingly blunt, opposing view. Indeed, for anyone interested in the process of architectural planning – use of natural light, the way that buildings integrate with their surroundings, the discrimination and competition in the profession – this is a pleasure to watch. Most beautiful of all the buildings is his last, the much-loved Capital Complex in Dhaka, Bangladesh. A paean to democracy, the people who work nearby are visibly moved by its symbolism and strength. Interesting also are the wonderfully proportioned, tranquil Salk Institute in California and the planned (highly controversial) synagogue in Jerusalem.

But it’s a delicate balance, this switching between the personal and professional search, one that mostly works, but occasionally falters. One section in particular misfired badly for me. Nathaniel visits an old friend of Kahn’s, a colourful character for whom the architect designed a music boat that could travel the world and open up to reveal a concert hall for use in harbours and on sea shores. Rather than explain his identity to the man off-camera, Nathaniel chooses to disclose the reason for his visit only on screen, provoking an emotional response. It’s a dirty trick; and while many documentaries hope (and try) to capture such dramatic moments on camera, this shameless engineering only cheapens the film-maker’s emotional journey and endangers the otherwise fascinating trajectory of the story. Such moments of mawkishness aside, it’s a dramatic tale, absorbingly told. The interviews with both the ‘other’ women in Kahn’s life – designer Anne Tyng and landscape architect Harriet Pattison, Nathaniel’s own mother – are very revealing, mostly because of the insights they provide into gender issues during the 1950s; and we also movingly witness Kahn’s three children come together for the first time to discuss their relationships, and how they can now move forward as a ‘family’, and perhaps consolidate their identity away from the shadow cast by Kahn the man and the myth.