The irreality felt in his recent documentary is only partially the fault of the tricky, dry-humoured Danish director Mads Brügger, who at times gives the viewer a distinct impression they are watching a mystery thriller.

The film starts slowly, grating through scattered and circumstantial evidence dug up over several years, and is tied together with the bit-part conceit of a filmmaker dictating the story to a typewriting African secretary.

Although sometimes jarring, it just about works to keep the plot moving forward and such theatrics contribute to a feeling of unrealness in the noir-styled, ostensibly non-fictional (sometimes you wonder), impressively thorough investigation into what happened when a plane carrying UN Secretary-General Dag Hammarskjöld crashed over Rhodesia in 1961.

Revelatory second half
In its second half, Cold Case takes several sharp turns with a series of revelations – just when Brügger claims to have lost all hope of further progress – as the story evolves from curious intrigue to sheer disbelief.

We follow him on an improbable journey that leads to a shadowy South African organisation that sought to weaponise the HIV virus against black Africans. He further questions the involvement of British and US secret services in Hammarskjöld’s untimely passing, showing how colonialism can continue to manifest itself long after empires are dismantled.

Brügger’s latest documentary sees him team up with aid worker Goran Bjorkdahl to offer two hours of explosive revelations inside an irresistible and dangerous international conspiracy. The half-satisfaction at the film’s conclusion belies a gargantuan effort from the pair to scrape away at the surface of a story that seemed impossible to break, and if its most conspiratory details are true, it could prove to be a startling discovery of warped foreign policy towards post-colonial Africa.