There's an elephant lurking in the room from the outset of Biosphere, in which two men are the last survivors of an apocalypse: how will humanity live on? Best friends Billy (Mark Duplass) and Ray (Sterling K. Brown) have only survived thanks to the ingenuity of Ray, who built the glass dome in which they live, insulated from whatever it is that’s keeping the sky perpetually black outside. But the dome’s protective glass increasingly needs patching up, and their last female fish (responsible for the continuation of their food supplies) has just died, setting their clocks ticking.

What happens next — to the remaining male fish and humans — is an astonishing evolution that speaks to the strangeness of nature, which will break its most rigid laws in pursuit of its ultimate goal: furthering the species. Biosphere undergoes a similar metamorphosis: while its zany twist (which we won’t spoil) seems to direct it towards gross-out bro-comedy territory, it transforms, surprisingly, into something more profoundly philosophical. Like the dome, Biosphere’s structure isn’t as solid as it could be — it often meanders — but, with its thoughtful meditations on gender, sexuality, and evolution in all its forms, it’s easy to forgive this quirky indie gem that flaw.

With such a claustrophobic setting and a relatively long runtime of 1 hour and 46 minutes, it would be very easy for Biosphere to feel suffocating — and quickly. Its surprising philosophical bent spins the film away from that fate, though, and the easy chemistry of its two-man cast (in spite of the complex emotion of their situation) pins it all together to give its wacky concept genuine heart and keep the film both light and engrossing.