In this episode, Queen Beryl is speaking with her new general, Nephrite. Nephrite tells her that Jadeite was a fool and that he will do better. All humans are controlled by the movement of the stars, and the same is true of their energy. He will read the stars to dicover which humans are at the peak of their energy. Suddenly another general, Zoisite, appears. In the dubbed version Zoisite is presented as a female, supposedly to avoid any controversy since "she" is actually a gay male in the Japanese version. There are a few instances throughout the series in which content was changed in the dubbed version to avoid homosexual themes. Perhaps this was a sign of the times and the original content may have been preserved had this series been released in later years. Anyway, Zoisite says that Nephrite is overconfident and that it could be his downfall. Nephrite tells him to go away, but Zoisite persists and says he can see Nephrite begging for mercy in front of Queen Beryl. Zoisite disappears in a whirl of flower petals and Queen Beryl says she leaves everything in Nephrite's hands.
In Tokyo, a crowd of reporters chases a young woman down the street asking if she thinks they can win their next tournament (it is not clear yet what they are talking about.) In the background, a dark energy appears in the sky. We see Nephrite at what looks like a small church-like building that he has constructed, reading the stars using his powers. He says that the proud Sagittarius, Chiron, is about to fire his arrow at the center of the milky way. His new target will be someone named Rui Saionji. It appears that Nephrite takes a different approach to collecting energy than Jadeite did, focusing on a single optimum target rather than collecting energy from groups of people.
As it turns out, Rui Saionji is a tennis player. Usagi and Naru watch her practice and Naru cheers her on, calling her "big sister." She explains to Usagi that Rui is not her real sister but that they have known each other since they were little. Rui is a year older than Naru, so she considers her an older sister to her. Suddenly Nephrite appears in a fancy red sports car, and all the girls in the crowd comment on how gorgeous he is, saying that he is the president of some company or something. He rudely/amazingly does a flip over the tall fence onto the tennis court, and tells Rui she needs to put more power behind her swings. He takes the tennis racket from her opponent and challenges Rui to a match. As Rui serves the ball to him, he returns it but uses his powers to do so, knocking the tennis racket out of Rui's hand and hurting her wrist in the process. She asks who he is, and he says he is Masato Sanjoin, an "eccentric coach." He goes to pick up the tennis racket for Rui, and uses his powers to command a monster to inhabit the racket and bring her to her peak energy level. He gives Rui advice to think of it as putting her weight behind the ball. When Rui takes the racket back, he eyes go blank and it appears something is wrong with her. "Masato" asks for a volunteer to step up as Rui's opponent, and a boy willingly obliges. Rui knocks him down with the tennis ball she serves and turns around to thank "Masato", but he is gone.
Nephrite says to himself that the more Rui uses that tennis racket, the more her energy will grow until it peaks. Rui is shown knocking down every one of her tennis opponents with every ball she hits. Late at night, Luna logs into the computer screen again at the closed arcade. The voice on the computer screen tells Luna that their enemies are part of an organization called "Dark Kingdom" but that it is still unknown why they want to collect human energy. Luna reveals this information to the girls the next day at Hikawa Shrine. Luna and Rei tease Usagi as usual and even go so far as to say it may have been a mistake that she is part of their group, but Ami sticks up for her. Usagi asks Ami to go play tennis with her, but Ami has cram school. Back on the tennis court again, Rui is still knocking people down and she seems to be acting meaner and meaner towards people. She even breaks one of her opponents' tennis rackets but commands her to keep playing. Naru comes to Usagi's house and says that she is worried about Rui - she never acted this way before. Usagi realizes that Naru is very upset at the way Rui is acting. Naru remembers the two of them playing together as children and how Rui always stuck up for her. She also recalls the present, in which she asked Rui what was wrong after the tennis matches. Rui told her it was none of her business and walked away. Usagi tells Naru that she will help her investigate what happened.
Late at night, Usagi and Naru go to the tennis school Rui goes to. Usagi thinks that she can get back at Luna and Rei if she does a good job investigating. The two girls discover that Rui is still on the tennis courts. She has injured two more boys and commands them to keep playing anyway. Naru and Usagi confront Rui and tell her to stop and that they are worried about her. Rui gets angry and attacks them with the tennis racket. Finally, her energy reaches its peak and the monster appears to collect her energy. Since Naru has been knocked out, Usagi uses the opportunity to transform into Sailor Moon and prove her worth. This is one of the first times when she shows some real character development and becomes less of a coward. Sailor Moon confronts the monster and gives her speech, but the monster generates a tennis racket and heaves a strange ball at her. Sailor Moon is trapped inside a giant tennis ball and is forced to bounce around the court. Tuxedo Mask interferes and stops the monster from torturing Sailor Moon. The monster throws a ball at Tuxedo Mask, but he dodges it and uses the bright overhead lights to conceal himself. He extends his baton and hits the monster with it as Sailor Moon is released from the giant tennis ball. Tuxedo Mask tells her not to let her guard down as the monster gets back up. Sailor Moon is happy that she gets to fight alongside Tuxedo Mask. The monster throws several fireballs at Sailor Moon and she tries to dodge them, but the last one looks like it is about to hit her after she falls down. Tuxedo Mask jumps in just in time and pulls her out of the way, flying upwards with her in his arm. He tells Sailor Moon it's time to finish off the monster, and they split off in two different positions. Tuxedo hits the monster in the face with a red rose and tells Sailor Moon to finish him off, but it appears that Tuxedo Mask is weakened for some reason. Sailor Moon runs to help him but he insists that she finish off the monster instead. She uses her Moon Tiara Action and defeats the monster, returning Rui to normal. Luna, Sailor Mercury and Sailor Mars run up to the scene but see that Sailor Moon has just defeated the monster without them. They admire her courage and Luna says it may mean she is growing up a little. Sailor Moon turns to find that Tuxedo Mask has disappeared and wonders if he will be all right.
The next day, Usagi and Naru talk about Rui. Apparently she does not remember anything that happened yesterday. Rui runs up and asks Naru to play tennis with her and the girls run off to the court together. Usagi daydreams that it would be nice to play tennis with Tuxedo Mask, but is interrupted by a stray tennis ball that hits her in the face. Naru apologizes to her, but Usagi falls to the ground.