A female Maiasaura finds a lost egg and decides to take care of it along with her own baby. Much to everyone's surprise, the child born from the stray egg is a Tyrannosaurus. Although the baby is a carnivore and his adoptive mother a herbivore, she is determined not to leave the newborn behind, so she abandons the pack and raises the two kids by herself. Named Heart by his mother, the Tyrannosaurus grows up together with his Maiasaura brother, Light. Soon enough, Heart realizes the difference between carnivores and herbivores; as a result, he makes the tough decision of leaving the family and discovering where he belongs. One day, Heart stumbles upon a baby Ankylosaurus. The baby mistakes him for his father, and accepts the word "umasou", which means "you seem delicious", as his actual name. Originally intending to eat him, Heart comes to be genuinely concerned for Umasou's well-being as a familial bond develops between them.