For a film that grapples with the legacy of colonialism, Marta Pessoa’s documentary begins with a rather provocative title card – it says: “Portugal is not a racist country.” This refers to the popular belief that overseas Portuguese imperialism – or “lusotropicalism” – is a gentler form of colonisation, compared with other European empires. To dispel such misconceptions, the film turns its attention to the Portuguese colonial exhibition of 1934, with the aim of exposing the racist treatment endured by its African subjects.

Taking place in Porto, the fair’s organisers erected a number of housing quarters to replicate native dwellings found in Portuguese colonies, in which Indigenous people were placed and where they performed their daily rituals to more than a million spectators. Seeking to extol the virtues of imperialism, this spectacle of colonialism amounted to nothing more than a cruel example of a human zoo.

Scouring through archival materials, Pessoa and co-writer Rita Palma pay particular attention to a photograph of a topless African woman, known only as Rosinha. In searching for Rosinha’s real identity, Pessoa returns again and again to this voyeuristic photo, all while showing other examples of Black subjects photographed in similar states of nudity. Unfortunately the effect is to inadvertently reinforce the racist gaze found in these ethnographic documents.

The film offers few alternatives to challenge these modes of representation; for instance, the voiceover includes a plethora of excerpts from colonists’ writings, but none from revolutionary Black authors. There are some brief scenes featuring interviews with and performances from Portuguese people of African descent, yet they are too few and far between to make a meaningful impact. The final result is not so much a reckoning with Portugal’s racist past but a rather inert, and at times tedious, projection of white guilt.

• Rosinha and Other Wild Animals is on True Story from 14 February.