A film about fantasies slipping away, Robin Campillo’s semi-autobiographical Red Island begins with a daydream, of a world of miniature buildings and puppet-faced men facing off against a masked girl. The girl is quickly revealed to be a visualization of Fantômette, the heroine of the popular Georges Chaulet book series that bears her name, and a particular obsession of Campillo’s 10-year-old stand-in, Thomas (Charlie Vauselle).

The film unfolds largely around a military base in Madagascar, from 1970 to 1972. It’s a decade after the island country’s independence from France, but various ties to the former colonial power remain in place, with French soldiers staying on their bases and working with the local troops. Perhaps inevitably, the oddly paradoxical Red Island is at once lackadaisical and urgent, relaxed but with a clear eye for how swiftly everything will end for the characters at its center.

Not that Thomas, peering...