A Yeti is convinced that the elusive creatures known as "humans" really do exist.
A yeti named Migo is convinced that a human known only as "Small Foot" is real and has to prove to his tribe that it does exist with the help of Meechee and the S.E.S - Smallfoot Evidentiary Society.
- The film opens with a yeti named Migo (voice of Channing Tatum) telling the history of his village. He and a large group of other yetis live high on a mountain above the clouds where they live according to a series of stones held by the Stonekeeper (voice of Common). According to the stones, the yetis were pooped out by a yak god and left on the mountain that is held up by ox that must be fed, or their village will fall into "The Great Nothing". Migo's father Dorgle (voice of Danny Devito) is the gong ringer who is supposed to summon the "sky snail" (sun) to bring the day. Migo and the other yetis are happy and peaceful ("Perfection").
Migo aspires to be the village gong ringer just like his dad. Stonekeeper calls Migo to practice with Dorgle for when his time comes. After a few failed attempts, Dorgle launches Migo toward the gong, but he gets distracted by Meechee (voice of Zendaya), Stonekeeper's daughter that Migo has a crush on. He flies over the gong and tumbles down the other side of the mountain. A plane then crashes down near Migo, and someone is ejected from it. Migo goes over to see the person and is surprised to see that it is a "Smallfoot" (human), considered by the yetis to be legendary. Before Migo can bring him back, the wind blows the man by his parachute and sends him down the mountain.
Migo runs back to the village to tell the others of his discovery. He brings the whole village with him to the site of the crash, but the plane slides down the mountain and falls over, and its imprint in the snow is blown away, leaving no proof of what Migo saw. Because there isn't a stone to prove that the Smallfoots are real, Stonekeeper accuses Migo of saying that the stones are wrong. Migo admits to disagreeing with the stones, so Stonekeeper banishes him from the village.
As Migo walks through the foggy snow, he is found by the yetis Gwangi (voice of LeBron James), Kolka (voice of Gina Rodriguez), and Fleem (voice of Ely Henry). They are part of the S.E.S. (Smallfoot Evidentiary Society, or "Smallfoot Exists, Suckas"), and because of Migo's claim, they take him to their leader, who turns out to be Meechee. Despite her father's beliefs that Smallfoots don't exist just because of the stones, she believes that there is something or someplace beyond their home ("Wonderful Life"). Meechee convinces Migo to go out and find the Smallfoot.
The yetis lower Migo with a rope to go down beneath the mountain. They are interrupted by Meechee's brother Thorp (Jimmy Tatro). In their distraction, Migo falls as the rope breaks and he lands in a bed of snow. He sees the forest for the first time and is astonished by the world outside the mountain.
Somewhere in town, we meet Percy Patterson (voice of James Corden), a wildlife documentarian trying to get a video on an exotic spider. His nature show has been struggling in recent times due to people being interested in other things. The man from the plane then comes running in a panic as he tells Percy that he saw a yeti. Percy talks to his assistant Brenda (voice of Yara Shahidi) about trying to get a video of the yeti to boost his ratings, as he has personally fallen on hard times ("Percy's Pressure"). He wants Brenda to get in a yeti suit so they can fake the video and make it go viral, but Brenda refuses, as she once respected Percy for his integrity toward animals.
Migo comes down near the bar where Percy is. Percy steps outside and sees Migo, but he thinks it's Brenda in the suit. He is impressed until he sees Brenda riding away with the suit on a snowmobile, and he realizes Migo is a real yeti. Percy freaks out and tries to tranquilize Migo, but he gets himself knocked out, and Migo brings Percy with him.
On the journey back to the village, Migo stops inside a cave when Percy gets frozen from the cold. He warms Percy up over a fire, though it appears as though he's trying to roast Percy as a meal. Later, they find that they are in the cave of a tired mama bear (Patricia Heaton). She complains to Migo about him and Percy disturbing her, and Migo tries to talk to her (which Percy can't understand), so it appears to him that Migo is almost saving him. On the way out, Migo gets his foot caught in a bear trap. Percy sees that Migo is hurt, so he bandages his toe up. Migo resumes the journey and accommodates Percy so he can stay warm.
The morning nears, and Dorgle prepares to do his duty, but he misses the gong, and his helmet breaks in two, but the sun still rises to his surprise. He rings the gong with both broken helmet pieces, and Dorgle is left questioning his job and purpose.
Migo returns to the mountain and reunites with Meechee, Gwangi, Kolka, and Fleem. They arrive in the village so that Migo may present Percy to the whole village. He is amazed to see the whole yeti community. All the yetis then become amazed and intrigued by Percy ("Wonderful Questions"), but Stonekeeper is not very happy at this discovery. Meechee suggests to her father that he talk to Migo to better understand each other. Meanwhile, Percy tries to explain human life to Meechee and the others through drawings, but he starts to feel a little sick.
Stonekeeper sends Thorp to bring Migo to their home. There, he tells Migo the truth about their history ("Let It Lie") - Stonekeeper has always known that humans existed because the yetis used to live below the mountain, but the humans' attempts to eliminate them drove the yetis to live high up. The cloud formation that obscures their view of the land below is a result of the yetis unknowingly dropping blocks of ice into a machine that makes steam so the yetis won't know of the world below, and the history with the stones was just something Stonekeeper made up to protect the others. He asks Migo to help do the same when Migo realizes that Percy did react almost instinctively to attack Percy.
Stonekeeper brings Migo in front of the other yetis as he holds Percy, who is suffering due to high altitude sickness and is growing colder and with less oxygen. Migo is forced to lie and say that he didn't find a Smallfoot, but rather a rare form of yak. Everyone is upset, but especially the S.E.S., who know that Migo is involved in a cover-up. Meechee in particular is very upset with Migo. Stonekeeper brings Percy to his cave.
Migo feels bad for betraying his friends once they abandon him. He goes to Dorgle, who encourages him to find Meechee. He launches Migo toward her home, but he finds that she isn't in her room. Stonekeeper hears Migo coming in and sees that Meechee is gone, and she's taking Percy back below the mountain.
Meechee's friends lower her and Percy down below, and Migo goes after her, along with Gwangi and Kolka, but Fleem takes too long to decide whether or not he should come down (because, according to Gwangi, "Fleem sucks"). They follow Meechee's footprints and see that she is headed into town. She leaves Percy somewhere warm, but she becomes distracted by the town's views, and the humans spot her and become afraid of her. Percy and Brenda find each other, and she tells him that the video he sent her from his time in the cave with Migo has gone viral. The police arrive and try to capture Meechee, but Migo and the others save her and they hide out in a yeti museum, where they find that the humans think they are monsters.
The yetis try to run back up to the mountain but are chased by a SWAT helicopter. Stonekeeper shows up and throws all of his stones to take the helicopter down (but not hurt those inside). Migo comes across Percy, who reluctantly shoots a tranquilizer dart at him. It then appears as though Migo has stumbled toward the police, and that they have shot him, but it's really Percy in the yeti suit to create a distraction, but now everyone thinks that he faked the yeti video. He and Brenda reconcile, but Percy is arrested for creating a disturbance and destruction of property. Percy then deletes all the pictures and videos he took with the yetis to protect their existence.
Migo and his friends go back up the mountain after they realize Percy saved Migo, and only then does Fleem come down when it's too late. With Stonekeeper's help, Migo speaks to the village again, saying that while the humans may seem terrified of them, and vice versa, the yetis should still give them a chance. Stonekeeper helps Migo stop the steam machines so that the yetis can finally see the land below them.
The yetis make their way toward the town. The humans gather, once again nervous and a bit panicked, but Percy walks through the police blockade, followed by Brenda, to prove that the yetis are not dangerous. The other humans soon follow, and the yetis start to have friendly interactions. Meanwhile, Fleem is the only one who has failed to make it down the mountain, until he gets scared off by a goat.
The closing credits show little drawings on stone of how the humans and yetis now play with and learn from each other.