Eager to grow up, Abigail "Abby" Louise Jensen (Debby Ryan) has been planning for her Sweet 16th birthday party ever since she was a little girl with a list of secret wishes she wants to come true. When the big day finally arrives, she excitedly adds her 16th and final wish to the secret wish list she's been keeping - a photo of Logan (Keenan Tracy), her crush. Then begins the first of many unusual occurrences, each including visits from a peculiar woman, Celeste (Anna Mae Routledge). She first appears as an exterminator when the Jensen's house gets overrun by wasps from a nest in their attic that has been building up for 16 years. The Jensen family are now unable to go back into their house until the wasps are exterminated, however, Celeste manages to extract Abby's wish list from the house. Abby's best friend, Jay Kepler (Jean-Luc Bilodeau), appears and offers Abby a jacket to wear over her pajamas, as she is unable to go back into her house to change. When Abby reaches into Jay's jackets pocket, she finds a birthday present for her from Jay. It is a necklace with a half of a heart saying "BFF". Jay has the other half which he uses a charm on his key ring. As Jay and Abby go to their bus stop, a delivery truck pulls up, and Celeste comes out, dressed as a mail woman and gives a package to Abby. When Abby opens it, she discovers 16 candles and a matchbox.
Unwilling to ride the bus to school, Abby lights the first candle and wishes for a new car, the eighth wish on her list. However, her first wish, meeting Joey Lockhart (Joel Semande), the hottest celebrity, is fulfilled. Abby then realizes that the candles correspond to her 16 wishes on her list and that they're each coming true every time a candle is lit. Celeste later tells her after midnight, all wishes will become permanent. Abby lands the attention of Logan, gets the Ford Mustang she's always dreamed of, she gets back at her lifelong nemesis, Krista Cook (Karissa Tynes) by beating her in a volleyball match, then getting elected as Student Body President, even though she didn't sign up for the elections. Also, she is suddenly fashionable, athletic, and popular.
During classes, Abby remembers that she needs the perfect dress for prom, and decides to go out and buy it, together with Jay, which agrees to pay for it. They are followed by Krista, who decides to see where Abby's sudden good luck comes from, and when Jay drops his wallet she takes it. She also convinces the store clerk that they are not going to buy anything (saying they are children) and are just wasting her time. The clerk believes her and kicks them out. While in the changing room, Abby uses the ninth candle and makes a wish to be treated like an adult, which instead, makes her an adult. This turns for the worse for Abby as it results in her not being allowed to attend high school anymore, and no one at school remembers her, not even Jay. Her parents also buy her a new apartment and leave her to live on her own.
Regretting her wish and lost childhood, Abby tries to look for a solution by going through her wishlist, though none of them are useful. goes to Krista's Sweet 16 and convinces Jay that they are best friends by showing him the birthday present he gave her which restores his memories of her. However, Jay is unable to help her and can do nothing. Desperate, Abby seeks the aid of Celeste, who is also unable to help her. Celeste then explains that there are several rules of magic, and one of them is that a wish cannot be undone. Abby eventually finds a loophole through the rules, seeing as how her last wish was a picture of her crush, glued on with gum, which seems to act as a "barrier" between the picture and the rules. She switches the picture for a picture taken that morning of her and wishes she could go back to the previous morning.
Abby's life then goes back to normal, along with some dead wasps in the process. She and Krista reconcile after Krista tells Abby why she hates her - back in the 3rd grade, Abby and Jay made a pact to be best friends forever, leaving Krista out, though up until that point, Krista and Jay had been best friends. Abby and Krista have a combined Sweet 16 party and Krista and Logan get together. Abby and Krista stop competing and work together to make the person who truly deserves to be Student Body President: Jay. Abby and Jay kiss before sharing a dance together. Abby says she got everything she wants; Celeste then disappears, smiling.
