This movie from Canadian director Megan Park starts very well; it is an apparent body-swap adventure with plenty of promised laughs featuring Aubrey Plaza, and her billed presence alone made it one of the autumn’s most attractive-seeming films. But mystifyingly, dismayingly, My Old Ass morphs into a gloopy, droopy, soupy young adult fantasy romance which finally fails to deliver its own ending, and we are denied the actual big finish, without which the entire story has been a pointless waste of time. (I suspect an earlier script-draft had that ending but Park had second thoughts.)

Park imagines a teen called Elliott (Maisy Stella), whose parents run a cranberry farm; it’s the end of the summer and she is about to begin university at Toronto, but is enjoying a glorious romance with a girl who works at a coffee shop. Then Elliott does shrooms with a couple of friends and has a wacky epiphany/vision: her supercool, cynical 39-year-old self, played by Plaza, appears to her. After a lot of freaked-out conversation, including an assessment of the relative firmness of their respective asses, and with much grumpy and adult-teen resentment from older Elliott at this enforced intimacy with her past self, she finally agrees to give young Elliott a piece of advice: stay away from a guy called Chad. And the next day, once Elliott has awoken from what she is now convinced was merely a lucid dream, she meets-cute with a guy called Chad (Percy Hynes White) who is super annoying but super nice and … well … you know.

Plaza’s natural toughness gives this film some texture, but the truth is she isn’t in it much. You can spend very, very long stretches of the running time longing for her to re-emerge. So, when she doesn’t, it feels bland.

• My Old Ass is in UK cinemas from 27 September