Pretty Guardian Sailor Moon Eternal is a two-part anime film covering the Dream Arc of the Sailor Moon Crystal adaptation.
During a solar eclipse, the short-lived peace the Sailor Senshi earned by defeating Master Pharoah 90 in season 3 is disrupted when a winged horse comes to warn Usagi, Chibiusa, and Mamoru about another old enemy of Silver Millennium returning to threaten Earth. The winged horse (erroneously referred to as a "pegasus," which is the given name of a winged horse in Greek mythology, not a species name) turns out to be Helios, the guardian priest of Elysium. In another part of Tokyo, the inner Sailor Senshi spot a flying ship that disappears at the end of the eclipse. The Dead Moon Circus is eventually revealed.
Part 1 covers Super Sailor Moon, Super Sailor Chibi Moon, Tuxedo Mask, and the inner Sailor Senshi gaining their super forms and confronting the Dead Moon Circus, specifically the Amazoness Quartet and the evil witch Zirconia. The true enemy, Nehelenia, Queen of the Dark Moon, remains mostly hidden.
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In a perfect world, the manga faithfulness and fully realized magical powers of Sailor Moon Crystal would be fused with the transformation sequences and character development of the 1993 Sailor Moon anime. Sailor Moon Eternal makes that wish no longer necessary. Part 1 is dedicated to answering critiques of Sailor Moon Crystal and the wider Sailor Moon lore in general. The inner Sailor Senshi are given dreams and their backstories are fleshed out. There is also more detail given to the transformation sequences of the Sailor Senshi.
Ami/Sailor Mercury is shown to harbor abandonment issues from her father leaving, and wonders if she will ever have love in her life. The enemy initially exploits this until Ami realizes she is surrounded by love from her mother, father, and all the other Sailor Senshi. She realizes she studies so hard to become a doctor and the best Sailor Senshi she can be because her dream is to help and protect everyone. To make memories with all of the people she loves. This self-reflection brings out her Super Sailor Mercury form. We also see her role as team strategist affirmed by encouraging words from Setsuna/Sailor Pluto, showing a more developed relationship between the inner and outer Senshi.
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This rounding out follows with the other three inner Sailor Senshi.
Rei/Sailor Mars realizes that her dreams are her own, and she is not bound to her grandfather's dreams and needs. Learning not to hesitate, and trusting her instincts (with encouraging words from Michiru/Sailor Neptune), brings out her Super Sailor Mars form. Super Sailor Mars' Flame Sniper looks amazing here. The animation is one of the best, right next to Super Sailor Uranus' World Shaking.
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We learn that Makoto/Sailor Jupiter lost both her parents early in childhood, and so desires to find love and start her own family as soon as possible. She also wants to run a bakery. And a flower shop. We learn that she has many dreams, including her dream to protect her friends and loved ones. She is Super Strength Jupiter, and needs to be at her strongest to get her dreams to come true. Her encouragement comes from her warrior spirit and relationship with Haruka/Sailor Uranus. This is how she becomes Super Sailor Jupiter.
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Finally, Minako/Sailor Venus is shown to have a dream of becoming a Pop music idol. As the leader of the inner Sailor Senshi, we also see that she takes her initial inability to transform the most seriously. She keeps this a secret from the other Sailor Senshi, and lashes out at them for bringing up the outer Sailor Senshi. This gives us insight into Minako/Sailor Venus' insecurities around being the leader of the inner Sailor Senshi. Regaining her confidence and connection to Artemis, Minako gains her Super Sailor Venus form.
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The lore is also expanded in Sailor Moon Eternal. The inner Sailor Senshi are revealed to have been princesses in their own right during Silver Millennium. We also learn that all Sailor Senshi and Guardian beings (like Prince Endymion/Tuxedo Mask) have their own personal sailor crystals similar to Princess Serenity/Sailor Moon's legendary Silver Crystal.
This is not to say that the movie is perfect.
Like many team based anime/stories, Sailor Moon suffers from an overemphasis on the title character and not enough emphasis at times on other characters. The respective team roles also sometimes feel contrived. The team strategist aspect of Sailor Mercury feels inconsistent. She pulls out her computer visor to say the obvious dimension distortion they are experiencing is indeed a dimension distortion only occasionally. Similarly, Sailor Mars and Sailor Neptune both wander into psychic captain obvious territory as well throughout Sailor Moon Crystal, both stating that obviously evil people are evil and that vaguely bad things are going to happen.
The ending of Part 1 was a bit underwhelming as well. Super Sailor Moon and Tuxedo Mask are put on a bus by a curse, and as the inner Sailor Senshi congratulate each other in the middle of a battle, they are captured by the Amazoness Quartet. Cue the outer Sailor Senshi having to come save the day predictably.
Overall,
Pretty Guardian Sailor Moon Eternal, Part 1 
