In the Late Cretaceous Period, a female Maiasaura finds an egg and decides that the baby in the egg needs to be taken care of, so she adopts it as her own. She takes it to her nest near Egg Mountain, a volcano with a giant egg shaped rock. One fateful day, a marauding pack of Troodon attacks the nesting grounds, snatching and eating the Maiasaura eggs. The outcome of the attack resulted in the mother losing all but two of her eggs (one of which is the egg she saved). As the eggs hatched, one was born a male Maiasaura, whom the mother named Light, and from the egg she found, the baby is revealed to be a Tyrannosaurus, named Heart. The herd leader was about to kill Heart in order to prevent him from eating anyone when he gets older, but the mother Maiasaura stops him, arguing that is new born and harmless. The leader then gives her the chance of leaving him in the wild where he belongs, and she was about to do so. However, she changes her mind out of guilt of abandoning Heart and leaving him alone, so she decides to move from the herd and raise her children by herself. Years later, Light and Heart now live in the forest with their mother, where they would be safe from predators and free from the herd. But Heart is unaware of his carnivorous nature and can only eat berries. While they were playing, Heart and Light felt from a cliff and a swamp, where they meet a sloth mother singing a song about tyrannosaurs and describing their appearances. The sloth sees Heart and retreats in fear, while Light joke about the appearance of Hearth, saying that he looks like the description in the song. His words makes Hearth sad and he start crying. When their mother find them, she grounds Light about what he says and he apolozige. However, despite the fact that Hearth asks their mother to don't blame Light, some questions start to mess up his mind.
As Heart is unable to live by eating plants and can't sustain himself of berries, he travels outside the forest and into the plains to look for food; unfortunately he watches a Triceratops fighting a pack of Tyrannosaurus, one of which is impaled by one of its horns and killed. Suddenly from the mist enters Baku, a particularly large Tyrannosaurus with a scar over his right eye, who slams the Triceratops to the ground, killing it. After watching the pack eat the corpse, Heart panics and tries to retreat quietly to avoid being eaten as well, not knowing he was one of them. However, one of the Tyrannosaurus named Gonza notices his presence and the fact that he smells like a herbivore. Baku ask him about his family, and Heart fearfully answered that he had a brother and a mother, not telling that they are Maiasaura. Then Baku explains the lifestyle of a predator, which makes Heart flee in panic, without noticing that Gonza was following him. After rejoining with Light, Heart tells in disdain his experience with the Tyrannosaurus, but Gonza arrives and threatens to eat Light. In panic Light yells to Heart to save himself, causing Gonza to wonder why a Maiasaura would take care of a Tyrannosaurus. Light explains that Heart is his brother, but then Gonza reveals Heart's real species, which Heart denies and attacks Gonza. He bites and rips off his tail, leaving Gonza to flee in pain, feeling betrayed. Unfortunately, upon eating the tail, Heart realizes that Gonza was telling the truth, and, upon tackling Light, that he couldn't control his predatory instincts, so he decides to run away to avoid killing and eating his family.
Years later, Heart is now a fully grown dinosaur with enhanced hunting and martial arts skills, such as beating a whole herd of Titanosaurus, and having a step forward from Gonza's pack. Meanwhile, Light is allowed to return to the herd as Heart doesn't live with them anymore. One day Heart discovers an egg which houses an Ankylosaurus. When the baby hatches, Heart tells him that he is "Umasou" (the Japanese word for delicious), and he prepares to devour him, when the baby jumps and cuddles Heart, believing that Heart was his father and that Umasou was his name, much to Heart's surprise and confusion. He then decides to raise him (initially to wait until he fattens enough for him to eat). Umasou acts very loving toward Heart, but Heart remains emotionally distant to Umasou as he wants to eat him later. Later Heart goes to hunt a Parasaurolophus, leaving Umasou waiting him in the grass. A nearby Chilantaisaurus notices him and is about to eat him, but Heart arrives and quickly sends it flying away with a kick. Umasou cheers Heart for saving him, but Heart angrily tells him that the Chilantaisaurus could have eaten him and to stay alert for predators. The next day, Umasou wants to gather berries to eat with his father, while Heart frantically searches for Umasou. He gives up the search and loses hope, but then Umasou returns with the berries. Enraged, Heart yells at Umasou to go away, which makes Umasou cry and tell why he left. Heart notices the berries and apologizes for yelling at him. Umasou mentions that he got the berries from Bekon, an elderly Tyrannosaurus who has grown too old to eat solid food and eats the berries. The two go to meet him and he explains Baku's story. A massive pack of Giganotosaurus arrived from the south and goes on a rampage, killing everything in their path, forcing Baku fight them by himself. The battle resulted in him losing an eye and receiving his scar, but he succeeded in defeating the Giganotosaurus and is now the king of the plains. After what happened that day, Heart decides to train Umasou to protect him from predators, and during the training, he started to like Umasou to the point of abandoning his initial plan of eating him. Even all the time he spent with Umasou, Heart still didn't accepted him as his son and was worried he might find out the truth, so he tricks him in having a race where, if Umasou wins, Heart would stay with him forever, and lets him run away from Heart. But Umasou enters the outlands where encounters Gonza's pack, who nearly kills him. Realizing Umasou was in danger, Heart went to the rescue and saw Gonza about to eat Umasou. Driven to a rage, Heart attacks Gonza and kept Umasou safe in his mouth, before defeating the gang with his bare martial arts skills. Then Baku appears when he witnessed what Heart did, and decides to banish Heart as a punishment for beating others of his kind. Once Heart got away from the plains, he is relieved and glad to see that Umasou is still alive and safe. He finally accepts him as his son and promises to live together, and they leave the plains to find their new home. While this time, Baku went to the meeting with Bekon and the old dinosaur start to discuss about the fact that Hearth look like Baku at his age. As Bekon begins to talk about Baku's lost egg, Baku cuts him off and says that belongs to the past.
Some months later, Heart and Umasou (who had grown slightly bigger) have moved to a beach. But Heart falls from a cliff and into the ocean, where he is saved by a female Elasmosaurus called Pero Pero. Pero Pero didn't know at first that Heart was a carnivore, until the two met and got attacked by a Tylosaurus, which Heart kills by ripping off one of its flippers. Yet, Heart promised to never hurt Pero Pero, and she won't be afraid of him. After Pero Pero realizes that Egg Mountain is having high volcanic activity and tells it to Hearth, he and Umasou go to save their family. On the way, as they cross Baku's territory again, their first obstacle is Gonza, who wants to get revenge for everything that Heart did to him, and he wouldn't let them pass until he's dead. The two have a very short fight, in which Heart wins by ripping a chunk off of Gonza's neck. As he lays dying, Gonza learns Umasou is Heart's son and ridicules this, telling Umasou that Heart is not his father. Umasou says that he knew this, but still considers Heart his dad. Knowing this, Gonza apologizes (and maybe forgives Heart). When Heart reaches Light's herd, his brother informs him that their mom doesn't live with the herd anymore and that she still is in their former home in the forest. He didn't look for her because of his job in the herd as guardian. However, the herd is too afraid of the volcano to move. Out of his deception, Heart starts to scare away all the Maiasaura to move to safety. After clearing the second obstacle, Heart rushes to encounter his mother, whom at last feels relieved and warm hearted upon finding her lost son. Heart introduces Umasou, and meets his new siblings, a girl and two male twins. After the warm reunion, all the family, including Light, head away from the mountain, whose eruption was strong enough to blast the egg rock from the volcano. They almost escape from the eruption, but Baku appears from the volcanic dust and threatens to kill Heart since he disobeyed his order of exile. In order to protect his family, Heart accepts a duel with Baku, in which Baku uses normal Tyrannosaur attacks and movements and Heart displays quick martial arts kicks. In the midst of the battle, Heart bites Baku's neck, which proves to be a great mistake when Baku grabs Heart, leaps, and smashes him with all his weight to crush him, almost killing him. Heart's mother pleads to spare him, revealing that she is his adoptive mother. This makes Baku wonder what would she do if Heart was starving for not eating meat, to which she responds that she would let him eat her, which Baku considers nonsense. He explains that a carnivore has to eat meat every day in order to survive. At the same time, he accepts that Heart wouldn't have survived without her help, so he agrees to spare Heart and the others. As Baku lies down to recover from his wounds from the battle, he tells Heart to be strong and good with a certain emotion (implying that Baku is probably Heart's father).
In the aftermath, Heart's Mom asked him if he would live with them when they reached their new home, but he refuses, telling that it would be better for everyone to stay with their own species. She agrees, guessing Heart would have wanted that, but he also denies, saying that he enjoyed being her son. The family cuddles and wave goodbye for last time before Heart and Umasou go on their way again. During the end credits, the Maiasaura herd move on to a new home. Baku returns to the plains and Bekon welcomes him back. The movie ends with Heart and Umasou watching the egg rock of Egg Mountain fly away to outer space.
Miyanishi Tatsuya Gekijō: Omae Umasou da na, the anime series, was scheduled to air on TV Tokyo beginning on October 4, 2010. The film opened in Japan on October 16, 2010.
Dinosaurs and other prehistoric reptiles featuredAchillobator (two individuals chased by Heart)Alamosaurus (cameo)Ankylosaurus (Umasou)Archelon (cameo)Avisaurus (background only)Chilantaisaurus (the purple horned carnivore, species implied in the second episode of the animated series)Elasmosaurus (Pero Pero)Giganotosaurus (seen only in Baku's backstory, referred to as "gluttonous long-necks from the south")Maiasaura (Light, Heart and Light's mother, among others)Ornithocheirus (background only, one is seen in the beach scene dropping a red berry)Ornithomimus (background only)Parasaurolophus (background only, one is killed by Heart)Protoceratops (seen fleeing from Heart, one is killed and eaten)Pteranodon (background only)Titanosaurus (the sauropod herd Heart defeats)Triceratops (one battles Gonza's pack and is killed by Baku, others appear in the background)Troodon (the flock of maniraptorans that raid the Maiasaura nesting grounds)Tylosaurus (fights Heart and is killed)Tyrannosaurus (Heart, Baku, Gonza, Bekon, among others)Velociraptor (burnt corpse seen during eruption)several neornithescorpse of a narrow-snouted theropod, possibly a spinosaur
Freelance writer Brian Switek said that the film was one of the strangest and "most adorable" dinosaur films he has seen.