Starring Michael Cera and Gaby Hoffman. In English and Spanish with English subtitles. Rated 18A.

Sometimes, the Ugly American can be a passive-aggressive postadolescent played by Canada’s Michael Cera or a deceptively love-happy flower child played by former child star Gaby Hoffman.

It’s not clear how Cera’s Jamie washed up in Santiago, Chile, but when we meet him, he’s planning a pilgrimage with easygoing buddy Champa and the local guy’s two younger brothers. (All three are siblings of the film’s writer-director, Sebastián Silva.) He’s determined to find and prepare a cactus called the San Pedro, which yields a kind of mescaline they hope to take at a remote beach. On the day of the trip, he doesn’t even remember having invited Hoffman’s character, Crystal Fairy, a gringa who is just as selfish around the natives as is the incessantly whining Jamie.

When she shows up by bus, he discourages her with smiling putdowns. When they all check in to a rough hostel while searching for that special cactus, he’s freaked out by her propensity to walk around naked (Crystal Hairy, he calls her), but the other lads don’t seem to mind. Once they get to their seaside destination, Jamie tries to control everything, but he has a rival. She has an advantage, because her new-age philosophizing fits better with their tripping. But as she presses the others to share secrets in their imperfect English, it becomes increasingly apparent that this spell-casting spirit isn’t quite as free as she seems.

Shot in less than two weeks and largely improvised—where dialogue is involved, anyway—the film leaves some questions unanswered and you can’t always tell if that’s by design or just because streams meander that way. The beauty of the breezily made tale, which offers the sweet sting of true experience, is that it hardly matters.