Ami chan begins by telling us a little about herself. She gets to school forty minutes early each day so she can read a book. She likes sandwiches because she can eat them and read at the same time. Her favorite subject is math, her favorite sport is swimming. But her least favorite thing is a love letter. she opens up her locker and sees a love letter propped up against her books. Her face turns read instantly.
Later on, Usagi notices that Ami's face is bright red. She asks what happened, but Ami says that it's only a rash from too much stress. To herself, Ami thinks that this has been the third love letter she's received, even though she doesn't think she should get them. To her, being a ninth grader is all about studying for entrence exams, not falling in love. [In Japan, high school is started in tenth grade] She flips through a magazine that publishes the scores students receive on mock entence exams. She lets out a startled "Eek!"
In first place is Mercury, Ami's pseudonym. Tied with her is a mystery person by the name of "Mercurius." She explains that this Mercurius has tied with her in every mock exam given. She's certain that Mercurius is a "he" and that he may be an enemy. The girls try to persuade her to go out with them over the weekend, but Ami wants to take another mock exam to try to beat Mercurius.
Some time later, the girls talk about how tired Ami's been looking lately. Minako explains that she tired because all she does is study and take mock exams every week. Her goal has become to out-score Mercurius, no matter what it takes.
When the results from the latest test are published, Mercurius has once again tied Ami for first place. Annoyed but impressed, Ami wishes she could meet this person who is her intellectual equal. Minako observes that Ami looks like a person in love, but Ami says that she only wants to meet a worthy rival. Usagi and Minako think that this sounds suspicous, especially if Mercurius turns out to be a boy. When asked what her ideal man would be like, Ami says that she'd like someone like Albert Einstein. Disgusted, Minako and Makoto turn away. "Who's that?" Usagi asks in earnest. Rei rolls her eyes more at Usagi's ignorance than at Ami's choice.
Later at the Crown Fruit Parlor, the girls discuss Ami's obession with Mercurius. They decide that this is her first crush and that they'll use Pasokon Tsuushin [the Japanese answer to AOL] to find the real identity of Mercurius. However, they acknowledge that the only one with enough smarts to figure out how to find Mercurius' screen name and trace his phone line is Ami.
The next day, Usagi meets Naru in the hallway, and they both go to the library where Umino is studying. Usagi gets a great idea: since Umino is taking the same mock tests as Ami and Mercurius, maybe he knows who the mystery person is. Umino does know Mercurius and offers to introduce him and Usagi.
At night, Ami goes to a new test prep school in order to become smarter than Mercurius. She sits down, but promptly falls asleep because she's been studying day and night. She wakes up at the end of class feeling sick. She wonders if Mercurius is doing this to her. As she leaves, a boy enters the room and signs up for the next mock test with the name "Mercurius."
A swirl of mist rises out of the prep school. "I am the genius loci Bonnoun," says the newly formed spirit. She's a spirit who likes to prey on students. When she sees Ami, Bonnoun knows immediately that Ami is in love.
At school the next day, Ami collapses and is taken to the nurse's office. She feels awful, as if she has the flu. Usagi's afraid that Ami has "brain strain," and Minako thinks it's lovesickness. The nurse explains that it's nothing but a cold brought on from not enough sleep. Everyone leaves to let Ami rest. "My head hurts. My body won't move," she moans. "I want to study, but I can't. What's wrong with me? This couldn't be your fault, Mercurius?" Bonnoun appears over her bed and attacks.
"So in the end you're ruled by your bad dreams, your passions!" Bonnoun says as she throws razor-sharp love letters at Ami, pinning her to the bed. "I'll hurry and make you a being of passion!" Ami wakes up with a start unable to get the letters off her. In her illness and exhaustion, she thinks that Bonnoun is Mercurius. Bonnoun tries to correct her, but Ami is too stubborn.
"You're such a coward, Mercurius! The time is finally here for our game! I can't lose!" An equation appears: "Exams squared plus prep school squared plus the square root of stress times temperature times disturbances squared over ranking equals rival equals enemy!"
Ami transforms into Sailor Mercury, all the while Bonnoun protests that she is not Mercurius. "Rival = Enemy! That's the Ami equation! This proves it!" Mercury uses her Mercury Aqua Mirage to defeat Bonnoun. After her victory, Mercury falls into Usagi's arms in utter exhaustion.
A little later, the girls laugh about Ami's "Rival = Enemy" equation. Minako kids that she'd better get studying, otherwise Ami might kill her. Ami asks the girls if they ever saw Mercurius because she never got to see his face. Minako holds up a post of a young idol and explains that Mercurius looked just like this. Ami is very pleased. "Just like when Eistein-sama was young!"
Usagi appears in a box with a picture of a boy with spiky hair and thick glasses, who is actually Mercurius. The girls are going to hide his true idenity because they don't want to spoil Ami's ideal image of her rival. The boy explains that he just spelled his name, Suuri Kurume, backwards [in Japanese, it would be Merukuriusu instead of the English Mercurius].
When the next results come in, Ami is again tied with Mercurius. "I was supposed to have beaten him!" she cries. "Why!? Damnit!"