Sailor Moon almost tricks its viewers with the first half of its first season. The first set of episodes is just so goofy, with the monsters of the week and their weirdly capitalist schemes to lure in young girls. It's almost hard to believe that the show will ever turn dark--and that's what makes what's to come all the more heartbreaking.
It's just Usagi and Naru, Usagi's best friend, at this point. They've been together since the first episode--and Naru is the one who seems to keep Usagi together.
Naru falls in love with Sailor Moon's newest enemy, Nephrite. (His human name is Masato Sanjoin). It happens over the course of a few episodes, and for the most part, she's unaware that Masato Sanjoin, the man she loves, is a bad guy and not exactly human. And probably way too old to be dating a middle-schooler. (By the way, one thing Sailor Moon does so well is honoring the--sometimes totally unrealistic--expectations of teenage girls, without mocking them.)
Anyway, when Naru does figure out he's Nephrite, she doesn't care. She shields him from Sailor Moon's Moon Tiara Action; she was ready to die for him. Naru's love has blinded her so deeply that she doesn't see that Nephrite has been manipulating her. Eventually, she gets captured by some folks who are after something Nephrite has: the Dark Crystal.
Despite being the bad guy -- and using Naru to get to Sailor Moon -- Nephrite seems to care a little about Naru. Afterall, he does go after her when Zoisite's henchwomen take her, saving her in what adds up to be more than a few times. When Naru and Nephrite are sitting under a tree, he admits to lying, but Naru doesn't care; she realizes that lies aren't always intended to be malicious. Sometimes they're kind.
After they've promised to eat ice cream together, Zoisite's Youma eventually find Naru and Nephrite and impale him with some magical death sticks. Naru tries to help him--and at one point, it looks like the ~power of love~ might actually allow it -- but the Youma attack again. Nephrite shields Naru and takes much of the blow. He dies right in Naru's arms, after apologizing for breaking the promise he intended to keep: eating those chocolate parfaits with her.
His body disappears, and all that's left is the bandage Naru ripped off her shirt for him and what appears to be Nephrite's green blood, all over her hands.
The Sailor Scouts are crying -- they showed up somewhere in the middle of everything -- and Naru's crying, and now we're crying.
Mamoru Chiba (ahem: Tuxedo Mask) has basically no backstory until this episode. In some ways, that actually makes sense -- he only recently figured out that he occasionally wears a cape and a top hat. Before his discovery, Mamoru transformed into his alter-ego whenever Sailor Moon was in trouble, unbeknownst to himself. It was hard to feel anything for the masked man with a tiny head.
That is, until this episode. We've known for a while that Mamoru can't remember anything about his past--just that the legendary Silver Crystal will help him regain it. And his memories aren't gone just because they can't fit into his abnormally small head -- seriously, it's alarmingly small -- but because he was in a tragic car accident as a kid. Both of his parents died and he got knocked unconscious. When he awoke, he knew nothing about himself. That's when he started having dreams about the mysterious woman who happens to have the same exact hairstyle as Usagi.
When Mamoru tells Usagi all of this, neither of them have any clue about the other's identity. Oh, but then, THEY DO. When they step out of the elevator, they're attacked by Zoisite and forced to either transform in front of each other or die. So they transform.
It's a magical moment (there's a montage), and everyone is shocked but happy. And then Tuxedo Mask gets stabbed by Zoisite. And is potentially dying. Mamoru is literally the closest he's ever been to figuring out his past -- and it's the first time the audience really gets to connect with him -- and he's seemingly about to die.
Sailor Moon is understandably upset about all this. A single tear harnesses the power of the rainbow crystals Zoisite and Tuxedo Mask were fighting over... and reveal that Sailor Moon is actually the Moon Princess everyone has been looking for. Then the episode ends. Cue an obnoxiously 90s Bubble Tape commercial.