Because disguises are classically much better than they appear to the audience in anime, Usagi didn’t know Chiba Mamoru and Tuxedo Mask were one and the same until she woke up in his house. But in this case, she kicks herself for not realizing it sooner, and despite Luna’s reservations, decides to believe in him.
Mamoru tells her his backstory, which is, like everything on this show, pleasingly straightforward: he was on a pleasant weekend drive with his folks when his dad suddenly decided to drive off a cliff (or maybe it was an accident). Mammy lost both his parents and his memories on that day, and thinks the LSC is the best way to get them back.
Usagi can’t quite explain why she’s trusting the dashing fellow so implicitly, nor does said fellow know why he told her so much about himself that he’d probably never told anyone else. But we know why: it’s because they’re falling for each other. That much is clear when having lunch with her friends, all Usagi wants to do is tell them about Mamoru; only she promised not to. She also stole his watch, and he nicked her hanky! Ah, love.
It gets clearer still when “Usako” (Mamo-chan’s nickname for her) decides the only way to heal the hordes of brainwashed citizens and classmates programmed to hunt Sailor Moon down is to become Sailor Moon. When she ends up in Zoisite’s clutches and the other guardians are knocked down, it’s Mamoru who springs to her rescue, right as she’s wishing for him to do just that.
The bad guy is stronger than usual, so even Mamoru needs saving; enter the real-life version of Sailor V, who’s been in contact with Luna for some time, whom I’m sure we’ll see a lot more of a couple weeks from now. If she’s the subject of the video game, then she was a sailor guardian before even Usagi. I wonder what her story is.
Finally, we also see a smidgen of nuance from Queen Beryl, who seems to regret awakening plotting her nebulous leader Queen Metalia, and plans on claiming the crystal—and the world—for herself once its found. Somehow I doubt ruling the world will be much fun for anyone, but hey, everybody’s gotta have a goal!