Arcadia of my Youth was a reboot. It started from scratch. It was then rebooted for Harlock Saga. Each time Harlock changes series everything before it is eliminated, which makes it easier to follow. Meanwhile 999 had contradictory lines at the same time, and keeps going back and forward with changes.
999 has roughly three continuities.
First you have the manga and tv series that are functionallly the same. There is a second manga adapted into the third movie.
Second you have the movie continuity, which appeared at the same time the show and manga were being made. Here Prometheum is related to La Metalle (from Queen Millenia), Tetsuro's dad worked for Harlock and sort of forgot his family, Emeraldas isn't Maetel sister, and in general everything revolves around Harlock.
Then you have the modern OVA one that mixes from both. For example, Emeraldas is back to being Maetel's sister, but Prometheum came from La Metale.
Most 999 spin offs (like the planetary shows) follow the manga logic, which means focusing on Tetsuro and Maetel visiting places no matter the context. |