Chikyuu no Kaijuu  

地球の怪獣, Earth's Monster, Monster of Earth

In order to safeguard humanity against extraterrestrial threats, a 52-year-old man named Daikichi Oogami must transform into a giant and engage in combat with bizarre alien creatures. Despite his heroic efforts, he has become the subject of ridicule and amusement among many people. His son, also named Daichi, is dismayed by how his father's struggles are being trivialized and overlooked. Determined to bring respect and gravity to these battles, Daichi decides to become a hero himself.