context: The plot was described by the author and numerous critics as a teen installment to a Jackie Chan plot with a Disney twist. Gary Marsh called the film "Buffy the Vampire Slayer meets Crouching Tiger Hidden Dragon".[8]
  Wendy Wu (Brenda Song) is an average, popular American teenager whose life is turned upside down by a visit from a young Buddhist monk Shen (Shin Koyamada). Shen claims Wendy is the reincarnation of an all-powerful almighty female warrior and the only one who can prevent an ancient evil spirit from destroying the world. Shen begs Wendy to wear an incredibly strong and powerful amulet that will protect her from evil until he can fully train her in martial arts.
  Appalled by Shen's obvious lack of fashion sense - he usually wears what looks like a bath robe - Wendy is too busy campaigning for Homecoming Queen against her school arch rival, Jessica Dawson, to be bothered with rescuing civilization. Wendy's traditional grandmother knows that Shen is speaking the truth; her mother (Wendy's great-grandmother) was the previous Yin Warrior who ninety years earlier defeated Yan-Lo in China. However, the others in Wendy's family have lost touch with their Chinese heritage. Shen's discussion of Chinese culture inspire Wendy's mother, a researcher at Fair Springs National History Museum, and Shen's mooncakes do the same for Wendy's father. However, faced with the choice between fighting evil and going shopping, Wendy is off to the mall in a heartbeat with her best friends Tory and Lisa. Yan-Lo soon materializes and sets out to destroy Wendy before she can attain her full Yin Warrior powers and abilities. In quick succession, the dark spirit possesses a security guard at the museum, Wendy's brother, her dog, her principal, her teachers, her best friend Tory and even Jessica Dawson. Wendy breaks up with her boyfriend, Austin after noticing how much of a jerk he is and starts to bond more with Shen. With Shen's help, her teachers are possessed by the souls of the Five Animals of Chinese martial arts to help teach Wendy. Mr. Medina becomes the Tiger, Coach Gibbs becomes the Snake, Mr. Tobias becomes the Crane, Mr. Garibay becomes the Leopard, and Shen himself becomes the Dragon. Wendy completes her training, and learns that she has become Homecoming Queen. She then discovers that the night she does battle is the same night as the Homecoming Dance. Feeling betrayed, she opts out. On the night of the Homecoming, her grandmother insists that Wendy fulfill her destiny, but Wendy refuses. She finally changes her mind upon learning from the monks that Shen has gone into battle alone. Wendy and the monks arrive to Shen's aid just in time. Wendy's martial arts training unleashes her inner heroine for a final showdown with Yan-Lo. In the last scene, Jessica thanks Wendy for lending her the Homecoming crown and the Queen status, opposing her usual hostile approach towards Wendy and the conflict between the two is apparently put to rest. Everyone is about to leave, but Yan Lo returns, this time in his actual form, and the battle continues. Shen begins to sacrifice himself, as it is his destiny, but Wendy saves him by changing his destiny. Wendy and Shen both kick Yan Lo together, and Yan Lo is destroyed forever. The monks tell Shen this is his last life, and they ascend. At the end, Wendy and Shen leave to get cappucinos and chocolate that Shen said he loves earlier in the movie, hinting that they might begin dating.
question: How often does evil return?
answer: ninety years