Sometime during the 2000s in the quiet town of Eastbrook, a once-bright high school quarterback named Hank Wallace sees his football dreams crushed not by a rival, but by a mysterious, plant-like parasite called the Ick. At first it seems oddly benign, creeping through Hank’s hometown as it slowly grows, unnoticed by a community that would rather focus on everyday life. Hank’s personal life unravels too when his girlfriend, Staci, leaves him for their classmate Ted Kim, setting Hank on a solitary road while the Ick quietly expands its reach.
Years pass and Hank stays in Eastbrook, taking up a different kind of responsibility as a science teacher at Eastbrook High. Meanwhile, Ted and Staci have married and built a realty business, seemingly thriving as the Ick continues to spread, invisible to most because its early behavior appears harmless. The eerie calm is punctured when Hank, while purchasing a rundown bar from the very couple he once dated, uncovers a troubling possibility: their daughter, Grace, who is also one of his students, may be his child. This revelation drives Hank to propose a paternity test using Grace’s saliva, a decision that will pull him deeper into a web of danger and doubt.
One night, Hank is assaulted by the Ick as it swarms an infected human, but his UV light therapy mask saves him by stalling the parasite’s growth. His research begins to sketch a grim pattern: the Ick grows in darkness but, when exposed to UV light, stops moving and goes dormant. The discovery is crucial, but the danger remains real as Hank narrowly escapes a barroom encounter with someone completely taken over by the Ick. Determined to warn others, he heads to a house party attended by Grace and her classmates, hoping to share what he knows. The crowd reacts with skepticism, dismissing his warnings, and the Ick soon crashes the party, infecting and killing several students in a frightening outbreak.
A military unit arrives and temporarily curtails the chaos, but their commander warns the town to stay indoors until the threat subsides, only to reveal that the unit is assigned to another town, leaving Eastbrook to fend for itself. The town’s residents largely ignore the warning, continuing with their routines, and the Ick rapidly infects more people. During a tense moment with her boyfriend Dylan, Grace is endangered, and Hank intervenes to help. Later, Hank is invited by Grace to ride home, but the journey becomes perilous when the Ick attacks again, forcing them to abandon the car after it overturns.
In the ensuing turmoil, Grace discovers the envelope containing her saliva sample—intended for the paternity test Hank had planned—inside Hank’s possession. She lashes out, accusing him of never having truly been part of her life, and she resolves that he will never be a father in her eyes. The prom becomes the film’s catastrophe point: before the big night, Hank confronts Staci about the test when tragedy strikes as Staci is pulled underground and killed by the Ick.
As the prom begins, Hank tries to warn the students, but the Ick has already infiltrated the celebration, consuming many students and even faculty, including the chaperone Ted. Hank fights his way to Grace’s side and rescues her from an infected Dylan, then escapes with Grace and her friends Heather and Griffin. The group hatches a desperate plan to use the school’s stadium lights to hold back the Ick, which has evolved into a towering, tentacled behemoth. The light weakens the creature, but it sinks underground and destabilizes the ground beneath the lights, forcing the survivors to climb a nearby radio tower to escape.
Sunrise comes, and with it a fragile beacon of hope: the Ick slides into a stasis as daylight surges over Eastbrook. The town begins adapting once more, installing UV lights around the streets to deter the nocturnal threat and allowing life to return to something resembling normal. Grace has begun to see Hank in a new light, accepting him as a father figure, and she reveals that she did in fact send in her saliva sample for the test. When the results finally arrive in the mail, the tension between certainty and doubt lingers, ending the film on a note of unease as Hank and Grace confront what lies ahead.
In the aftermath, Eastbrook clings to a cautious routine built around UV-lit protections and new vigilance, while the Ick’s threat is receding only by the slimmest margins. The town’s resilience is tested, and the bond between Hank and Grace grows into something steadier, framed by the knowledge that their relationship—and the mystery of Grace’s paternity—will continue to haunt them as they navigate a world where light, both literal and symbolic, is the only safeguard against a patient, insidious darkness.
