It's the only part of the manga that, years and years and reread and rereads later, still doesn't make any sense to me. 

In act 55, StarS 6, before Usagi goes off to fight Galaxia with the Starlights, she leaves Luna, Artemis and Diana with her mother.

Fastforeward to act 56, the next chapter. The group gets attacked and thrown into the sea of memories by Sailor Lethe, where Usagi awakens and is untansformed on a park swing set. Unable to remember anything, the three cats approach her and are simply mewing. Lethe later kills the three of them in one blast-as proven by Chibiusa in the same chapter. She feels an enormous pain and vision upon Diana's death, which leads her and the Amazones Quartet to fight by Sailor Moon's side.

My question is this; how the hell did the three of them end up in the sea of memories? If it really was a construct of Usagi's memories, how could killing them in that location have lead to their death in real life. Again, it is cannon that they (or at least Diana) died, considering Chibiusa's reaction in the 30th century. 

Or, in that moment, was Usagi literally back at that park, bringing Lethe with her? Were the three cats teleported to the river the exact moment Usagi started to forget? They couldn't have done it with their injuries caused by Tin Nyanko in act 54.

Actually, the last we see of them in the manga is the shot of them getting killed. Why didn't they show up at Galaxy Cauldron with the others once Chaos was defeated? Are we to assume they were revived as well, even though we didn't see them? Or, because they were not sailor senshi and had no sailor crystals, were they killed permanently? Or was Naoko simply too busy wrapping the manga up that she just forgot to write/draw closure for them, and they were revived?

Does anyone have any ideas on this? It's been bugging me for years.

Knowing Naoko she probably forgot about them.

That still doesn't explain the circumstances surrounding their deaths though.

Maybe Chibiusa only reacted that way because the cats died later in the timeline, not at that exact instant. Maybe that was the timeline where Sailor Moon lost and Earth was subjugated by Chaos, who would have probably killed the cats anyway. Usagi averted that death by defeating Chaos instead of giving in, so it never truly happened. As for why they aren't at the wedding scene... cats don't go to weddings? XD Just spitballing ideas.

Honestly there probably isn't a perfect in universe explanation for what happened, but I really doubt the cats are actually dead, they just aren't in the scene.

XD We had a huge thing on tumblr about this around thanksgiving of 2012. People had two views - that their death was totally in her imagination (but then, like you said, no kitties at all later. No other mention, not even at their wedding or in any of the wedding artbook shots), and that Lethe had been able to attack them via Usagi's strong feelings about them, reaching through that connection to kill them.  Like you said, it was their death that sent ripples through the future, to clue Small Lady into the problems happening in the 20th century, so... I think it was real. 

I mean, sure, Naoko could have just forgotten about them, but I like to think more that she had intended for them to die and for us to only really realize that later, when all the dust settles. In the first arc she had intended the senshi to die, sacrificing their lives to save Usagi, but she was denied that - they wouldn't let her do it. I think killing off Luna, Artemis, and Diana was under her editor's radar and she was still able to have that bit of reality. 

OH LOOK I FOUND TUMBLR STUFF:
When RedundantThinking brought it up (and haha my defense of NO THEY'RE ALIVE being shot to pieces and then i fell to pieces whee)
--When I fell to pieces I seriously fell to pieces and wrote this late at night while sobbing on the couch over thanksgiving ♥ : Goodbye (GC link) (tumblr link)
Then a question and answer:



so yes. I like to think they're dead, because it adds gravity to the story and just broke my heart. C:  I mean if they're alive then yaaaaaaay, but sweet sweet angst makes demands.

Then why would Chibiusa's vision occur the exact same moment they were killed, and contain the exact way Diana died?

Also you have to remember, they're not actually cats. They'd obviously have a longer lifespan and live well on into the beginning of Crystal Tokyo, at least.

There's this artwork:




That looks the royal bed to me. I like to believe they are alive.

But that could also be a stylized version of Mamoru's bed:


And the trio were known to have sleepovers in the SuperS arc ♥

When do they get married exactly? I thought they got married after Usagi becomes queen.

It's before, I'm pretty sure; in the last chapter when they get married they're not King and Queen yet. She recalls a dream, and that dream is a sunlit picture of crystal tokyo. When they get married she's wearing her Neo-Queen Serenity tiara, but there is no sign that Crystal Tokyo has already been built and they've ascended the throne. ((though, technically, they already had their coronation after the death of Nehellenia? ...but her wedding is the first time we see her crown in her lifetime))

I see...

hmmm the cats will always be alive to me. XD

Yeah, Uranus, it was right before she became queen. 

Then again, Luna and Artemis lived long enough to make Diana. Depending on which theory of time travel you look at, they could be alive.

Well technically speaking, Diana has to be reborn through the Galaxy Cauldron. She's the only heir to the Sailor Crystal of Mau. ;-)

Tin Nyanko isn't the legitimate inheritor, and planet Mau would appear to have been destroyed during Silver Millenium (head canon).

http://www.thegalaxycauldronforums.com/t9989-theorysailor-mau

Thanks for contributing to my debate thread! However, you're not allowed to double post, as in post multiple times one after the other. You're two posts took place within a minute of each other. In the future, just edit your first post to add what you want to add.

My apologies.

I like to think that they're alive.