I wasn't at all ready for how affecting and well made this movie was.

The premise of a kid meeting her older self would have been at home in the corporate studio scene in the 80s, and even though I've got almost 20 years on star Aubrey Plaza, the idea of meeting my younger self and warning him about all the disappointments, broken dreams and failures, hoping to put him on a better path than I went down, resonated deeply.

That said, I didn't expect much beyond some hijinks about drugs, sex, jokes about old age and some kernel of authentic truth that might emerge near the end.

It was nothing – nothing – like that. When Elliott (Plaza) folds herself into the bemused Chad's (Percy Hynes White) arms and a single tear rolls down her face, it's the first time I've misted up in a movie for a very long time.

18 year old Elliott (Maisy Stella) lives in the Muskoka region of Ontario where her family farms cranberries and her life has been one of harvests, gumboots and flannel shirts, gunning around the waterways in her tinny and spending time with her friends Ruthie (Maddie Ziegler) and Ro (Kerrice Brooks).

Elliott's mum, dad and two annoying younger brothers love her but she can't wait to leave. As we meet her, sneaking off to camp on an island in the river with her friends to drink and do shrooms, her departure to live and study in the city is fast approaching.

Out and proud, Elliott tells the bait shop girl how she feels and hooks up with her, ignoring her mother's texts wanting to know where she is, the rest of the family sitting at home waiting to celebrate her birthday.

But Elliott, Ruthie and Ro sit around a campfire talking shit, deciding how much shrooms to do and enjoying one of their last times together.

Ro and Ruthie react to the drugs in their own way but Elliot's is something else again. All of a sudden a middle aged woman (in the form of Plaza) is sitting on the log next to her, staring calmly at the campfire, her friends having disappeared. Elliott freaks out while the woman tells her she's her older self.

It's not terribly clear how it happened or whether Elliott's older self has some purpose in being there, but when the younger woman gets over the shock she naturally wants to know everything about how her life turns out.

The older Elliott gives her some mild tidbits of wisdom – it's soon obvious her life isn't anything like she wanted it to be – but she exhorts the younger woman to appreciate her life and family more and to not be so desperate to grow up and move away. One rule in particular is clear; stay away from anybody named Chad.

Elliott wakes up next morning hung over but with her new friend's number in her phone, the name of the contact 'My Old Ass'. As she texts back and forth with her 39 year old self she tries to take her advice to heart, spending time with her brothers, trying to listen and understand what she means to her mother (another scene, of the latter describing trying to put her to sleep as a baby, had me crying as well, just like Elliott does).

But it's when swimming at the local waterhole she meets an intriguing boy her age – intriguing because she's always been gay but finds herself drawn to him. Initially horrified because his name's Chad, she can't really avoid him because he's working in the farm community for the season, and when he offers to help her clean up and sell her boat, they're going to spend even more time together.

While she falls in love with Chad, Elliott desperately tries to contact My Old Ass, who hasn't answered any calls or texts for weeks, to find out what's so wrong with him. When the latter does finally show up one last time, the prognosis for her relationship with him is damming.

The first, least flashy aspect of how great the movie is is the world building, and not how you think. The life and times Elliott lives in – cranberry farming in the Canadian wilderness and river country – feels very real and very lived in, and I wouldn't be surprised if writer/director Megan Park grew up in similar circumstances.

If she did, she's rendered it with honesty, life and beauty you can feel in every frame. If she didn't and it's all made up, she's as brilliant a writer as she is a director.

Where the world building doesn't necessarily extend to is the rules around the astral/time travel element of the two Elliotts being able to meet. When her older self shows up for the final visit there's some reference to it being possible because of the influence of drugs, so you might think she's imagining the whole thing.

But they can talk on the phone and text each other, and when Chad happens by the pair talking during the older Elliott's last visit, he can see and talk to her as well. What exactly are the rules around the seemingly supernatural? In a lesser movie (or the dumb 80s teen comedy version) it might have mattered more, but I didn't care in the least – it was a vehicle to enable the premise and the gorgeous characters and story did all the work.

Then there's Stella, playing the young hero. One of my biggest bugbears is when movie types talk about how important characters are before they serve us up another iteration of the same tired old archetype. It's a very tricky balance between creating someone with their own life and having the audience relate to them, but when it's done right (like here) it's a thing of beauty.

I know I must have fallen in love with the portrayal because all the affectations of an 18 year old girl (calling everyone 'bro' and 'dude', using 'like' three times in every sentence) didn't bother me, and usually that stuff drives me nuts.

Like all real humans she's a constant contradiction, as strong as she is lost, as smart as she is flighty, swearing like a sailor but with a heart as big as the world.

And none of it would have been possible without Stella's talent. She's as natural, comfortable and real in the role as I've seen in a performer of her age, making it all look effortless. I never would have dreamed I could relate to a girl in her late teens but I felt every flutter and plunge of her heart.

And her performance is also a beautiful double act with Park's direction. Despite the sci-fi premise there's a real verite, naturalistic style to the acting, dialogue and the way Park lights and shoots scenes as well as holds and points her camera at them. I recognised a rare talent and immediately put her other movie on my list.

I remember trying to think about just one thing that didn't work in Chris Nolan's Inception – in fact I lead my review of that film with it – because I found myself in a similar position here, constantly surprised and delighted with a movie that remained almost perfect in tone, direction, script and performance throughout.

So I found two things to balance it out. There's a bizarre drug trip later on where she finds herself as Justin Bieber performing on stage that didn't seem to have much purpose beyond commenting about what drugs can do to you, and no matter where the characters are – in the middle of a vast lake, sitting on logs deep in the forest – they had flawless mobile phone reception.

Other than that, the superlatives I could write about it don't do it justice.