Note: This story, as one might assume from its headline, contains major spoilers for “Parasite.”

Staircases are an evocative symbol in Bong Joon Ho’s “Parasite,” a biting drama critics have named one of the year’s best films. One set of stairs descends to the central Kim family’s dingy semi-basement apartment in modern-day Seoul, while another ascends to the front door of the ultrawealthy Park family, for whom the cash-strapped Kims work. A third leads to the upper floor of the Parks’ stylish home.

And finally, as we discover when the tone pivots from that of a heist comedy to a rather harrowing — but still witty — thriller, there is the secret set connecting the Parks’ basement to a bunker where the husband of the family’s former housekeeper has been hiding from loan sharks for about four years. These steps are involved in many of the film’s pivotal moments, including those in the gruesome final act.

Staircases are among the many things in “Parasite” that could be considered “metaphorical,” as becomes a refrain of the Kims’ son, Ki-woo (Choi Woo Shik). Presented with the opportunity to tutor the Parks’ teenage daughter at the beginning of the film, he recruits his sister, Ki-jung (Park So Dam), father, Ki-taek (Song Kang Ho), and mother, Chung-sook (Chang Hyae Jin), to masquerade as unrelated workers so they can oust the Parks’ current help, infiltrate the household and take advantage of the family’s surplus wealth.

Though “Parasite” cannot be confined to a single genre, it centers its focus on class struggle, a sort of precursor to the warfare of Bong’s 2013 thriller “Snowpiercer.” The staircases represent divisions of class and power — the Kims wonder upon wandering upstairs whether, in a just world, they would also belong among the rich, so far above ground; upon descending to their flooded apartment in a storm that merely throws off the Parks’ camping trip, the Kims are reminded of the disparities between the two families.

But unlike so many upstairs-downstairs films, there’s nuance to Bong’s depiction of the lower class, largely because of the addition of the secret bunker. The original housekeeper, Moon-gwang (Lee Jung Eun), displaced by the scheme, turns up at the home while the Parks are away and begs the Kims to allow her husband, Geun-se (Park Myeong-hoon), to continue living there after they discover him. She calls Chung-sook “sister,” at which the Kims scoff. How could they possibly compare to a couple leeching off the Parks like this?

The argument between the two families becomes violent, with the Kims temporarily coming out on top. Over the course of a few pulse-quickening scenes, and unbeknown to the Parks, Chung-sook critically injures Moon-gwang by pushing her down the stairs, and Ki-taek traps Geun-se in the bunker.

While attending a birthday party for the Parks’ young son, Da-song (Jung Hyeon Jun), Ki-woo heads to the bunker carrying a heavy scholar stone he once received as a gift intended to bring his family material wealth. Ki-woo can’t seem to let go of the decorative rock, a symbol of his constant yearning for stability. Geun-se, driven mad by his own situation, smashes Ki-woo over the head with the rock and runs upstairs to the backyard, where he then stabs Ki-jung. This causes Da-song to faint, and Mr. Park (Lee Sun Kyun) immediately demands that Ki-taek drive them to the hospital instead of tending to Ki-jung.

The Parks are not immune to Bong’s scrutiny — though the mother (Cho Yeo Jeong) is portrayed as a truly kind woman whose naivete befalls her, Mr. Park, her husband, can be quite horrible — but this is a moment of clarity for Ki-taek, who realizes his family will always be seen as less than by people like the Parks. Grabbing the knife Geun-se wielded, Ki-taek stabs Mr. Park and runs away.

The film continues with Ki-woo’s narration. After waking up in the hospital, he and his mother are charged and put on probation. His sister is dead, his father’s whereabouts unknown. Acting on impulse, Ki-woo heads to a mountain near the house the Parks have since vacated, and which now belongs to a rich German family. He deciphers a message, delivered in Morse code via a flickering light, that his father now inhabits the bunker.

“Parasite” has a cynical view of how class operates in a capitalist society, but as Bong has said, it’s also painfully realistic. Geun-se worshiped Mr. Park and his professional success but continued to live beneath him. Ki-taek, who acted upon his anger, still remains beneath another rich family. On that fateful day, he ran from the backyard with blood on his hands — not Mr. Park’s, as one might assume, but his own daughter’s. Their quest for upward mobility is futile.

“Parasite” concludes where it begins, in the semi-basement apartment. Ki-woo pens a letter to his father, fantasizing about his plan to go to college, secure a well-paying job and buy the Parks’ old house so that his father can live freely. Bong shows us this vision: Ki-taek ascends the bunker stairs and embraces his son.

But rather than leaving viewers in this dreamy state, Bong pinches them and returns to the apartment. Ki-woo hasn’t yet executed the vision, recalling his father’s earlier advice that the only way to fulfill a plan is to not make one at all. Today — and maybe forever — the Kims remain underground.

“Parasite” is now playing.