Horror movies are often cloaked in the night, relying on the darkness to unsettle and scare.

Writer-director Ari Aster takes a completely counterintuitive approach by having Midsommar take place in midsummer in Sweden, when the sun never fully sets.

Neither is there an obvious grotesque-looking antagonist, just plenty of Swedes dressed in innocuous white for the festivities, with some of the women wearing flowers in their hair and others playing musical instruments. It all looks so idyllic, like something out of a fairy tale.

And yet, there is clearly a dark undercurrent.

An early scene of a car on the road is slowly tilted until the image is upside-down; a simple manoeuvre, but one that is effectively unsettling.

Aster, who made his feature debut with the acclaimed horror film Hereditary (2018), goes for psychological unease rather than cheap jump scares.

He lets the tension build slowly, almost casually, thrusting the group of unsuspecting college students into an unfamiliar situation and letting some off-kilter details register - a sacred temple where the visitors are not supposed to go is painted in a cheerful yellow; a bear is seen kept in a wooden cage; a folksy woven depiction of a love potion shows pubic hair as a key ingredient.

The film-maker also plays with the trope of the ugly American who is culturally insensitive. As if to compensate, the characters refrain from criticising or even questioning too much - until the tipping point is crossed.

The restrained approach pays off and the revelations that follow make quite a shocking impact.

Pugh (Fighting With My Family, 2019) and Reynor (What Richard Did, 2012) are believably real as a couple and the dynamics of their relationship - she leans on him for support, but does not want to be seen as being too needy; he has been thinking of breaking up with her and can be quite distant - add another layer to the proceedings.

Despite initial appearances, this is no dreamy tale, but a midsummer's nightmare.