In Dinosaur, Aladar is first shown as an egg. A few moments later, a Carnotaur attacks the herd and Aladar's egg is abandoned. Just then, an Oviraptor steals the egg and it and another Oviraptor brawl over it. As they brawl, they drop Aladar's egg into a river. As it is swept away by the current, a Pteranodon picks up the egg and it carries it far away to an island. Two birds pester the Pteranodon and it drops the egg. The lemur inhabitants find the egg and it soon hatches. The lemurs name the baby Aladar and he grows up.
One evening, just after the lemur's mating ritual, Aladar and the lemurs notice comets falling from the sky. Then, suddenly, a large asteroid crashes down. Aladar and four other lemurs (Yar, Plio, Zini, and Suri) escape the debris. Aladar utters a mournful roar for all the deceased lemurs. They soon wander through a barren wasteland when they are distracted by a passing Velociraptor. As they inspect it, other Raptors appear and start to chase them, but they manage to escape. Soon after, they encounter a herd of various dinosaurs (Iguanodons, Parasaurolophus, Styracosaurus, Pachyrhinosaurus, Microceratops, Ornithomimus, and Stygimolochs) led by Kron and his assistant Bruton. Aladar also encounters Neera, his love interest and Kron's sister, and three other dinosaurs at the back of the herd named Eema (Styracosaurus), Baylene (Brachiosaurus), and Url (Ankylosaurus).
Later in the film, the herd migrates to a far-off lake but when they get there, the lake is dried up. Kron musters the herd while Eema wanders to the barren lake and collapses. As Aladar comes to help, he hears rumbling under Baylene's footsteps. He and Zini dig a hole and ask Baylene to press hard on it, and sure enough, water comes out. Although Aladar stated there was enough water for everyone, all the dinosaurs in the herd push and shove each other for a drink, but were soon backed off by Kron who claimed the water for himself, and didn't allow any of the herd to have it.
Early one morning while the other dinosaurs were asleep, Aladar gets some water for some young orphan iguanodons which attracts Neera. As they get some water, Kron wakes up the rest of the herd as Bruton reported Carnotaurs nearby. The herd moves so quickly that they leave Aladar, his family, Baylene, Eema, and Earl behind. While walking that night, they encounter a wounded Bruton. They leave him be (as requested by Bruton) and then discover a cave as a thunderstorm starts. Later, Bruton joins them and two Carnotaurs attack the cave. Bruton sacrifices himself as the others escape deeper into the cave. This also kills the female Carnotaur and the male leaves.
While going through the cave, the company encounters a dead end. However, they see light bursting through a hole in the dead end wall. After they all break it, they find the Nesting Grounds. Aladar is pleased, but soon wonders where the herd is. Eema shows Aladar where she and the others used to come in the valley. The normal way in was blocked off by a rockslide, so Aladar goes back to warn the herd.
On his way back, Aladar sees a dead Stygimoloch. Then, a Carnotaur comes and it eats the dead dino as Aladar runs away. Aladar soon reaches the herd as Kron is about to make them climb the rocks. They fight and as Kron is about to strike Aladar, Neera intervenes by ramming into Kron and knocking him to the ground. She then walks beside Aladar as they lead the herd to the Nesting Grounds, leaving Kron behind.
The Carnotaur then comes and the herd begins to panic. Aladar says that if they scatter, they'll be eaten. then he tellls them to stand together. As Aladar faces the Carnotaur, Neera and the other dinosaurs in the herd join in. Roaring at the Carnotaur, they force it back and as Aladar keeps roaring at it, the rest of the herd walk past safely.
The Carnotaur then turns its attention to Kron amd it goes after him. Neera and Aladar follow close behind. Kron runs up the rocks and finds a cliff as the Carnotaur attacks. It throws him against a rock and is about to finish him when Neera starts to shove the Carnotaur. It shoves Neera back and as its about to get her, Aladar comes in. The Carnotaur pushes Aladar, but the resilient Iguanodon doesn't give up. He pushes the Carnotaur to the edge of the cliff. The ground beneath it crumbles and the Carnotaur falls to his death.
After Aladar and Neera mourn for Kron's death, Aladar leads the rest of the herd to the Nesting Grounds. After saying, "Welcome home," the herd rushes past Aladar and Neera in joy.
Sometime later, Aladar and Neera become mates and have a nest of their own. Their first egg hatches as the lemurs, Eema, Baylene, Url, and a couple of orphaned Iguanodons that Neera had looked after come over to greet the hatchling. After the baby hatches, Aladar roars in joy, followed by his friends, and finally the whole herd.
Aladar is compassionate and merciful. He is against Kron's ways of "only the strongest survive." Therefore, he does everything he can to help weaker dinosaurs such as Eema and Baylene. He loves his lemur family, referring to Yar as his grandfather, despite their species differences. He also acts like a big brother to Suri and Zini. Aladar falls in love with Neera, Kron's younger sister, who is considerably more compassionate than her brother. Despite his dislike of Kron, Aladar displayed sorrow when Kron was killed by a Carnotaur.
Aladar also seems to be a natural leader, which fueled his rivalry with Kron who feared he was trying to take his place as leader of the herd. When the herd was attacked by a ferocious Carnotaur, Aladar encouraged them to "stand together."
Aladar can also fight when the situation demands it, being able to hold his own against the larger and more powerful Kron by using his superior speed. He is also strong enough to push a Carnotaur over the edge of a cliff.
Aladar show similar to Tarzan as they both raised by a different species of animals and end up falling in love with a female of their own kind.