Oh Mal-soon, Na Moon-hee a 74-year-old widow, lives with her son and his family. Foul-mouthed, stubborn, controlling and shameless, she has a difficult relationship with her depressed daughter-in-law Ae-ja, Hwang Jeong-min but is very proud of her son Hyun-chul, Sung Dong-il who became a university professor of gerontology at Seoul National University and whom she raised by herself against all odds.
One day, Ae-ja gets hospitalized after suddenly collapsing. Her doctors strongly advise her to live apart from Mal-soon. On the day she is told by her son that she is being sent to a nursing home, the dismayed Mal-soon wanders the streets and comes across a mysterious photo studio that claims it captures the moments of one's youth. She takes what she believes will be her last self-portrait for her not-so-far-off funeral, but when she comes out of the studio, Mal-soon is dumbfounded by her own reflection in the mirror: she's now a fresh, young 20-year-old woman.
At first not knowing what to do, she hides from her family and soon ends up lodging with her fellow café shop worker who used to be a servant of her family. With no one recognizing her, Oh Doo-ri, Shim Eun-kyung decides to make the most out of this once-in-a-lifetime opportunity. She renames herself Oh Doo-ri after her all-time favorite actress Audrey Hepburn, and gets a haircut similar to Hepburn's in Roman Holiday, and takes advantage of her youth. As the "suspicious girl" of the Korean title, all anyone sees is that Doo-ri speaks with a regional accent and walks like an old lady, and that she acts very strangely and motherly for her age. After wowing a group of pensioners with a stirring karaoke tune one day, she attracts the notice of her unknowing grandson Ji-ha, Jung Jin-young and Han Seung-woo, Lee Jin-uk, a TV music producer. Ji-ha invites her to join his heavy metal band, and after she convinces them to switch to more audience-pleasing melodies.
She gets caught romantically between two men: Mr. Park, her now ex-café worker friend who had always secretly loved Mal-soon, and Seung-woo, who's doing his best to track her down again. Mr. Park's daughter forces her out as she thinks Doo-ri is scheming after his money and she takes refuge in Mr. Han's apartment.
Meanwhile, the band becomes successful with her as vocalist and Seung-woo decides to take them under his wing. Doo-ri is thrilled to be pursuing a career in music — something she had to give up when she was young. They are booked for an extravaganza at the studio and Ji-ha is late due to a traffic jam. He decides to leave the taxi and bike but is hit by a lorry on the way there. He is in a critical condition at the hospital but Doo-ri decides to go on stage so as to tell him that his song was a success afterwards. As she arrives at the hospital it is found that only she has the correct blood type for a transfusion. She does the transfusion knowing that it would return her back to her aged condition.
Ji-ha's band takes off with the sister as the lead singer and Doo-ri as a fan. Mr. Park finds the same photo studio and becomes young and handsome. He picks her up at a bus stop and rides off bickering good naturedly as they always do.
