Sixty-two years after human life on Earth was annihilated by rampaging alien invaders, the enigmatic Messenger O is sent back in time with a mission to unite humanity of past eras—during the Second World War and ancient Japan, and even back to the dawn of the species itself—to defeat the invasion before it begins. However, in a future shredded by war and genocide, love waits for O. Will O save humanity only to doom himself?