I saw a very convincing argument online that this movie makes an interesting double bill with The Substance, and It's hard to disagree – the theme behind the story here is very much the lengths we'll go to and what we'll put our bodies through in a bid to keep belonging, with our youth (in the case of The Substance) or with the person we love.

Real life couple Alison Brie and Dave Franco are Tim and Millie, still in love but obviously harbouring resentment about something in their past or relationship when we meet them at a party celebrating their move to the country away from their city life.

In short order we learn what's wrong. Tim, holding onto the dreams of musical stardom he's had since he was a much younger man, is mourning the death of his ambitions while trying to keep them alive. And Millie is frustrated that he can't grow up and fully invest in their lives together, including their long-dead love life.

But they move into a rambling old mansion, Millie to a new teaching position at the local school, and try to start fresh. She befriends her colleague Jamie (Damon Herriman), a teacher who lives close by and who'll become a confidante as things fall further apart with Tim.

The turn of events is precipitated by a hike in the nearby forest, where the pair get lost and end up falling into a hole that contains a smallish cave full of weird artifacts and a pond in the centre. With no apparent way out they have no choice but to sleep there all night, drinking the water and hoping for the best.

But the water has powers/is cursed, depending on your point of view. Almost immediately when Tim and Millie get home, weird things start happening that seem to point to a weird phenomenon where they're supernaturally connected – physically. They've already found their skin stuck together after waking up the hole thanks to what they assume is some weird fungus, but it gets worse.

Things start to get really strange, including Tim gripped by a sudden lust for his girlfriend, crashing her workday at her school and the pair make urgent love in the kids' toilets, Millie thrilled the spell is finally broken but horrified when it's over and Tim is physically stuck inside her.

And while the weird happenings build and get weirder and scarier, both are going through their own angst-ridden doubts about themselves and each other.

The truth about what's really going on with the pool in the cave, Jamie and the history of the weird cult that existed in the area is soon revealed, and it's a body horror premise that would make David Cronenberg clap his hands with glee. The title, like in the best examples of the genre, seems simple but tells you a lot more about what's happening.

Almost everything is narratively clear except for a few of the more outward ideas, and the tone by writer/director Michael Shanks does a beautiful job of keeping you wrong footed enough to know something's going on.

I was also surprised (though not unpleasantly so) about how gross and gory it gets. When I watched trailers around the time the film was first out I assumed it was a domestic thriller with some creepy elements, but it goes full alien/monster prosthetic makeup at times, and qualifies as a bona fide horror movie.

All shot in country Victoria, and the effects are top notch owing to Shanks' background in VFX, it's a weird, nasty little delight.