- "The last to hatch among his siblings and the smallest, 11-year old Arlo has yet to make his mark on his family's farm. Though he is eager to help out with chores, this sheltered Apatosaurus just can't seem to get past his fear of everything. So when Arlo falls into a river and finds himself swept hundreds of miles away from home, he's forced to face his fear for the first time in his life, braving the harsh, untamed wilderness and a long, arduous journey home. Along the way, Arlo develops an unlikely friendship with a human boy named Spot, who helps him realize he is capable of much more than he ever thought."
Arlo is the last and the smallest of the three children of Poppa Henry and Momma Ida to hatch out of his egg right after his sister, Libby and his brother, Buck; despite having hatched from an egg bigger than the first two. When shy Arlo comes out of his egg to meet his new family, Buck maliciously enjoys inflicting pain on by spanking him with a log; much to his father's anger. He spends the first day of his birth playing around the house with his parents and siblings.
Years later, Arlo has grown older and is in charge of feeding the family's chickens in their pen. But Arlo has one problem; he suffers from pantophobia, which causes him to have an abnormal fear of everything including chickens. Nevertheless, his mother tells him to feed them anyway. At first, all goes well when Arlo is somewhat bonding with one of the chicks in the pen until he is confronted by one of the adults, which causes him to run scared. Poppa Henry catches Arlo before he can unintentionally fall into the river after running away scared from the chickens. To make him feel better, Henry tells him he doesn't have to like them just feed them.
When Poppa Henry finishes building the corn silo to protect it from critters who might eat from it and makes his mark with his wife, Arlo and his siblings try to do the same but their father reminds them that in order to make their marks, each one has to earn it by doing something big, bigger than themselves; much to Poppa Henry and especially, Arlo's eagerness.
A couple of more years go by and after his siblings earn their marks on the silo for all their good hard work, Arlo, still pantophobic, is the only left with a mark unearned and Momma Ida seems rather concerned with his phobias. His fears, however, get them best of him, which cause him to mess up his and everyone else's chores too. On the day Arlo tries to really earn his mark without fear, Buck messes and teases him when pretending to have been killed by the chickens Arlo has been charged to feed for years. When Buck discourages Arlo and calls him a coward, his father scolds him and comes to Arlo's defense even when Arlo almost gives up.
To encourage Arlo not to give up on his dream of earning his mark, his father takes him out one night to show him how to overcome his fears and have fun with fireflies. In order to give Arlo a sense of purpose, his father puts him in charge of guarding their silo against critters and helps him set up a trap. While Arlo is on guard, the trap manages to capture a feral caveboy but Arlo doesn't have the heart to kill him and sets him free. Heavily disappointed, his father has Arlo accompany him to track the caveboy through a ravine where it begins to rain. After Arlo injures himself, Poppa Henry, feeling regretful for how he was mad at Arlo and realizing what his obsession with catching the critter led to, decides to turn back just as a flash flood occurs and a massive rapid appears, but Henry only manages to save Arlo before he is swept away and killed.
After his father's tragic death, Arlo is charged with helping his weak mother to gather the corns in their field to have enough food for the upcoming winter. When Arlo starts filling up the silo, the caveboy is at it again with the corn, which results in Arlo chasing him all the way to the river while blaming him for what happened to his father and getting themselves washed farther away from Clawtooth Mountain.
After Arlo is washed up ashore far from home, Arlo still insists on finishing off the caveboy. When Arlo tries to survive on his own with minimal success, the caveboy finds him and eventually gives him some berries to eat after trying to feed him with a live lizard and a bug after helping him survive out there. When Arlo wants to know the location of the berry tree, the caveboy leads him to it, where they are both attacked by a snake, which the caveboy fights and saves Arlo from, which amazes Arlo.
Then, a nearby Styracosaurus named Forrest Woodbush, also impressed with the caveboy's physical prowess, tries to call him by name, but he only responds to Arlo, who names him "Spot." After traveling together for the rest of the day, Arlo laments about his lost family, and confides in Spot, who reveals that his own parents are dead. They sleep together that night.
The next day, a storm ravishes the land, and Arlo, remembering the storm that took his father's life, runs for safety on his own and Spot is unable to keep up.
The next morning, when Arlo wakes up to find Spot, he encounters a group of Pterodactyls led by Thunderclap, who pretends to be friendly to Arlo; recounting to him his story of how he overcame his fears. However, his true motives become apparent when he eats a small fox whole that Arlo helped to free. Thunderclap and the others then set their sights on Spot. Arlo and Spot run away from the carnivorous Pterodactyls before encountering a pair of T-Rexes named Nash and Ramsey, who both ward the pterodactyls off and whom Arlo tries to run away from as well when they come straight at them.
Just when it seems that the T-Rexes are approaching to finish off Arlo and Spot as well, they actually reveal their true good nature when Ramsey is seen helping Arlo up. Nash asks if he's okay, Ramsey expresses her dislike of bullies and scratches Spot's chin after cuddling beside her foot. Then, the two T-Rexes begin to somewhat fight until their father, Butch arrives to break up the fight. Then, Butch turns to Arlo and tells him he's got no business being out here.
When Arlo says he's lost and he's looking for a river that will lead him back home on his farm, Butch states there are many rivers out there and asks which one he's following. Arlo tells him he's looking for the one that leads to Claw Tooth Mountain. Nash and Ramsey offer to take Arlo to a water hole somewhere South, where he may find someone with information on how to get back to his farm. But Butch states that he and his family simply don't have time to "play baby-sitters" and that they are on their way to look for their lost herd of longhorns. Ramsey also tells Arlo that Nash lost their herd, which Nash denies and simply states that they just "wandered off." However, Butch and his kids later agree to help Arlo when Arlo offers to help them using Spot to track down their herd. At first, when they see Spot having supposedly led them all the way to a bug, which Spot later eats, they seem disappointed and Butch supposedly threatens Arlo that "if he's pullin' his leg, he's gonna eat his" until Spot calls them with a bark to show that he found the herd's tracks, a dead longhorn, and a feather belonging to the ones, in other words, "rustlers" as Butch refers to them as, that stole them in the first place.
Once they find their herd, Butch instructs Arlo to lure the rustlers out. So, Arlo and Spot get on a rock in the middle of the herd and Arlo screams real loud when Spot bites him on the leg since Arlo proved to be a failure at it first, which attracts the rustlers, who are revealed to be a group of Velociraptors led by Bubbha. Before the Raptors can attack them, Butch and his children come to the rescue and try to fend them off. Arlo successfully attacks two of them; allowing Butch to scare them off for good. Then, the T-Rexes, Arlo, and Spot let out a few roars over their victory on the raptors.
At night, Nash offers Arlo to trade Spot for his bug harmonica, but when Arlo politely declines, Nash eats it. Then, Spot listens as the group shares their stories on how they got their scars when the T-Rexes notice Arlo's scar on his right leg due to his last battle with the raptors around a campfire. Nash got into a fight with 15 Stegosaurus outlaws where one of them got his spike tail stuck in Nash's right leg and pulled it back out, and since then, he is unable to feel his toes (demonstrated by setting them on fire). Ramsey got her tail stuck between a rock and a hard place when a stampede of longhorns was coming right at her, so she chewed the tip of her tail off to get loose. But both of them enjoy hearing their father's story. Butch once walked for five days without water at 100 degrees in the shade until he saw a pretty pond, so, he bent down to take a drink when suddenly three crocs launched out of the water and one of them bit him on the face but Butch wasn't ready for dying that day, so, he bit one croc in half, tail-whipped the other, and finally, drowned the last one in his own blood. Nash wishes he could touch the croc's tooth, which his father has stuck in his gums as a souvenir, but refuses to allow him to touch it. When Arlo states he is done being scared, Butch tells him that fear is something no one can "beat or outrun but can get through it and find out what they're made of." When Arlo notices the first snow meaning winter is coming, the T-Rexes fulfill the rest of the deal by promising to help take him back home in the morning.
The next day, as the T-Rexes are herding, Arlo and Spot prove a big help when they manage to bring back part of the herd that was intending to separate from them. Then, Arlo sees Clawtooth Mountain and the T-Rexes wish him luck on his journey with Spot. During the journey, Arlo and Spot enjoy playing with seagulls and then howling until they hear another howl from a distance. It is revealed to be a caveman, whom Spot tries to approach until Arlo takes him back. They ignore him and continue on since Arlo intends to keep Spot as a pet at home.
As another storm begins to pick up along the way, Thunderclap and the pterodactyls return and attack the two, this time managing to take Spot away and leaving Arlo behind. As Arlo begins to give up, he has a vision of his father leading him away and back toward home, but Arlo admits his love for Spot and resolves to save him, which makes his father proud before he fades away. Filled with determination, Arlo attacks the pterodactyls, who have cornered Spot at the river in a log, and the two of them manage to throw them; including Thunderclap, whom Spot bites a hole through his left wing when Arlo roars at him as a distraction, thus, rendering him slightly flightless, into the water where they ride helplessly downstream. A flash flood begins and a massive mudslide crashes into the river and triggers a torrent. Arlo leaps to save Spot and the two of them are carried away towards a waterfall. Arlo manages to swim to Spot and hold on to him as they both fall, with Arlo carrying Spot to shore where he regains consciousness.
As they continue their journey, Arlo and Spot both hear the calls of the same caveman and his family just as they come over the hill overseeing Arlo's farm. Spot seems to bond very well with them, hinting that the family wants to adopt him, but Spot still insists on going with Arlo. Nevertheless, Arlo honorably allows Spot to return and stay with them. Then, the two of them share a tearful, but loving goodbye with one last howl to one another as Spot and his new family disappear over a hill.
When Arlo returns fearless and triumphant, his family welcome him back home with hugs and watch him finally make his mark on the silo.
- Lucas Neff was originally cast to voice Arlo. However, following the director change and revamping of the film, he was replaced by Raymond Ochoa.
- An early report in June 2013 said that the name of the protagonist dinosaur was "Jacob" (pronounced "Yacob"). This was not supported by the subsequent report, and the official announcements at D23 Expo 2013 called him "Arlo".
- Concept art released before the replacement of Bob Peterson by Peter Sohn depicts Arlo as much larger than he appears in the final film. He also has blue eyes, a longer plate-like mouth, a light yellow snout and deeper nostrils.
- A toy Arlo can be seen on the floor of one of the scare simulators in Monsters University.
- In Inside Out, Riley has a memory of when she and her family look at what appears to be the Cabazon Dinosaurs, and the dinosaur which their car accidentally runs into resembles Arlo.
- Arlo is the first Pixar protagonist to lose a parent. Nemo does not count, as he is actually the tritagonist of Finding Nemo.