One day, a marauding pack of Troodon attack the nesting grounds, snatching and eating the Maiasaura eggs.
The herd leader plans to kill Heart in order to prevent him from eating anyone when he gets older, but the mother Maiasaura stops him, arguing that he is a newborn and harmless.
While they are playing, Heart and Light fell from a cliff into a swamp, where they meet a sloth mother singing a song about tyrannosaurs and describing their appearances.
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.
Light yells to Heart to save himself, causing Gonza to wonder why a Maiasaura would take care of a Tyrannosaurus.
He tackles Light, unable to control his predatory instincts, and decides to run away to avoid killing and eating his family.
Years later, Heart is a fully grown dinosaur with enhanced hunting and martial arts skills, which he demonstrates by beating a whole herd of Titanosaurus, and having a step forward from Gonza's pack.
He decides to raise him, initially to wait until he fattens enough for him to eat, but he gives up this scheme after a bad dream about his old family.
The two go to meet him and he explains that one day, a massive pack of Giganotosaurus arrived from the south and went on a rampage, killing everything in their path, forcing Baku to fight them by himself.
The battle resulted in him losing an eye and receiving his scar, but he defeated the Giganotosaurus and is now the king of the plains.
But Umasou also notices that he didn't catch him in time, so Heart finally accepts him as his son and promises to live together, and they leave the plains to find their new home.
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 declares that he won't let them pass until he's dead.
After clearing the second obstacle, Heart rushes to encounter his mother, who is relieved and warm-hearted upon finding her lost son.
After apologizing to her for leaving so suddenly when he was young, 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.
[1] Discotek Media released Heart and Yummie as a limited edition Blu-ray collection with only 2,000 copies total.