"A Journey Through 600 Million Years of Natural History" is set in a time when time machines have been developed, allowing access to ancient times. Inochi and Akari, a pair of university students, travel through various eras and encounter undescribed species of ancient life. The story depicts their "600 million year journey through natural history" from ancient times to the present day, gazing up at the giant dinosaur Diplodocus in the Jurassic period, having a barbecue on the beach in the Cretaceous period and eating ammonites.