‘Diary’ of a teen set loose in the unhinged ’70s

Past eras always seem safer than they actually were because we survived them, either personally or as a species, but the 1970s held some real dangers, and “The Diary of a Teenage Girl” brings them back with truth and specificity.

Unmoored from both traditional values and the loopy idealism of the 1960s, the ’70s’ prevailing moods were chaos and moral confusion.

Aside from its period accuracy, the most refreshing thing about the film is its moral complexity, which will probably make it controversial.

If you’ve heard anything about this movie, you know that it’s about a teenage girl who has a sexual relationship with her mother’s boyfriend (Alexander Skarsgard).

There’s nothing good to be said for a guy who would do such a thing, but the character, as presented in “The Diary of a Teenage Girl,” is not exactly a sexual predator, nor a pervert with a thing for underage girls.

The second decade of the 21st century finds almost everyone a moral arbiter in possession of absolute truth, but in the ’70s people were humble enough to be vague, with results both bad and good.

[...] if the film were flat-out erotic and titillating, that would also be a problem, and writer-director Marielle Heller would leave herself open to the accusation that she’s made an exploitation piece.

[...] Heller has the courage to include a striking scene in which Minnie, while having sex with a boy her own age, suddenly takes charge in a way that shows that she’s learned something from her encounters with a 35-year-old.