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?