Joshua Oppenheimer is optimistic. The filmmaker says his postapocalyptic musical film The End, starring Tilda Swinton, Michael Shannon and George MacKay, is an opportunity to spark change.

Speaking to The Hollywood Reporter at the San Sebastian Film Festival, where his movie is screening, Oppenheimer described being motivated to make this film after visiting the bunker of an oligarch who, perhaps eerily, refused to discuss exactly why he was investing in a bunker.

The End tells the story of a wealthy family living in their palatial bunker 25 years after environmental collapse left the Earth uninhabitable. Mother (Swinton), Father (Shannon) and Son (MacKay) are confined to the space and soon find themselves struggling to maintain hope and a sense of normalcy by clinging to the rituals of daily life. The family pushes harder to deafen the noise of creeping guilt — breaking out into show tunes about “a bright future.”

“When you’re...