In an alternate but identical universe, May 27 is Last Day, celebrated around the world as the final day of humanity’s existence … every year. The origins of this annual worldwide holiday are buried in myth, but no one really believes the world will end on any particular May 28. After all, there are mattress sales, beer-soaked festivals, and free tattoos dispensed at a Boston shop.

This particular May 27 starts at the International Space Station, where two astronauts — one Russian, the other American — and a Japanese space tourist complete experiments, communicate with their respective mission controls, and cope with their profound individual dissatisfactions. The only happy traveler is Yui, the Japanese space tourist, whose primary occupation is his own contentment. Bear and Svec, the American and Russian astronauts, are preoccupied with their orbital duties, as well as family issues on Earth, the lovely blue orb always visible from the ISS portholes.

Back on the blue orb, Kathy is overcome by her many psychological issues, including the inability to control her actions, overeating, and consuming inedible objects. Abandoned early in life by her mother, she’s obsessed with finding her half-brother, Dennis, with whom she feels an unshakable bond, and her only friend, Rosie, a Filipino immigrant who introduces her to a splinter congregation of disillusioned Jehovah’s Witnesses. Kathy anchors herself to Rosie, never sensing that the older woman is far from a safe harbor.

Meanwhile, Sarah, the beloved but benignly neglected high school daughter of two college professors, stews in ennui as she struggles to make a memorable human connection on this particular Last Day. Months after a few moments of flirting at a faculty picnic, she tracks down the ex-boyfriend of one of her parents’ colleagues, convinced they can create a more authentic world … even if she’s not sure what that means.

And as the frantic but common activities of another May 27 wind down, something off-key creeps in, hinting that, just like morning fog on a Massachusetts beach, something hidden has emerged in the last hours of this Last Day. It stretches its fingerlike tendrils as far away from Planet Earth as possible, to a cabin on the International Space Station, in the early hours of May 28, bringing with it what no one ever expected.