“Day One” takes its sweet time getting to the inciting incident (alien invasion) and instead sets up the plight of its heroine Sam, a terminal cancer patient spending her final days in a Brooklyn hospice that seems to be populated mainly by seniors. Sam is played by Lupita Nyong’o, an expert at reaction-driven horror performances, having played dual roles in Jordan Peele’s “Us.” She eventually gains a partner on her journey, a university student from England named Eric (Joseph Quinn of “Stranger Things,” and multiple BBC programs). Other characters appear in the movie, including a protective father played by Djimon Hounsou (who also appears in “A Quiet Place: Part II”) and a hospice worker played by Alex Wolff. They either get eaten by monsters or don’t. 

All are given a full measure of humanity in the brief time they’re onscreen. But the focus is on Sam and Eric, who bond with each other, each taking pity on and helping the other when things are dire or terrifying. What differentiates this film from the first two is that the main characters have no blood ties. It’s not about protecting your family but protecting somebody you didn’t even know five minutes ago, which is a different proposition. Sam and Eric randomly cross paths and become a dyad, moving towards the South Street Seaport, towards a large body of water (the creatures hate water) where boats might take them to safety. 

Sarnoski’s plotting refines the idea explored in the first two films, that we’re essentially watching zombie movies with xenomorph-style aliens in them who hunt by sound: aka, a survival film with a picaresque narrative. In stories like this, what happens to the characters during their journey is more important than the journey, or the destination. Which is not to say that the destination is unimportant to Sam — a Harlem pizzeria where Sam used to enjoy her favorite New York slice after watching her late father play jazz piano in a small club — but that “Day One” lets us know that it’s all a pretext to let Sam have scary adventures, test herself, and learn to care and love again, and to let the two lead actors act mainly with their faces and bodies, not so much with dialogue (because if the creatures hear you, they kill you).