Sedovia, a kingdom in Central Eastern Europe. The year: 1500. Princess Kasia, second in line to the throne after her older brother, Crown Prince Karol, has just been born, and the citizens of the capital are celebrating the royal birth. Among those citizens are 3-year-old Jaroslaw Jakubczyk and 3-year-old magic user Zoja Kowalska, who have met each other for the first time.
Jaroslaw and Zoja grow up together, and agree to marry each other one day. At age 18, Jaroslaw is drafted into the military for a war, gets volunteered for a suicide mission, survives, and gets to (read: is forced to) marry Princess Kasia as his reward, with the wedding to be held the next day, Kasia's 16th birthday.
Kasia, very unhappy about her sudden engagement to an uncultured commoner, packs a sack and escapes the castle in the night to find her true love elsewhere. The next morning, Zoja learns about her sweetheart's betrothal to Princess Kasia and goes to the castle with Jaroslaw's parents so she can say goodbye to Jaroslaw for the last time, only to learn that the wedding had been called off because Kasia had run away - Zoja realizes something bad is about to happen to Kasia. Sure enough, Kasia is struck by a mysterious bolt of lightning, and disappears without a trace.
Chicago. The year: 2000. 2-year-old Connor Pytleski, the son of a Sedovian-American man and an Englishwoman, is told the story of Kasia by his mother, and can't help but wonder what really happened to Kasia. 16 years later, 500 years to the day after Kasia's disappearance, she arrives in a park in Hyde Park, and is immediately mobbed by a crowd of young children. Screaming, Kasia runs away from them and into the street, and an 18-year-old man sees her in the street and slams on the brakes, stopping inches short of hitting her. Kasia faints immediately after witnessing what a close call it had been, and the young man, realizing that this young woman is impeding the flow of traffic, takes her and puts her in the front seat of his car.
20 minutes later, Kasia wakes up, finding herself in a bed, and sees the young man who almost ran her over. The young man introduces himself as Connor Pytleski, and it slowly becomes apparent to Kasia that she's not in the 16th century anymore. Connor decides that, because he can't send Kasia back and she has nowhere to go, he may as well try to get her used to modern life. Kasia also meets Connor's 10-year-old sister Emily and 13-year-old brother Peter. Connor realizes that his parents probably wouldn't approve of Kasia living with them, so he has Kasia stay in the den in the basement.
Back in the 16th century, Jaroslaw asks Kasia's father, King Marek, if he can marry Zoja instead now that Kasia is gone. Marek demands that Jaroslaw look for Kasia. Jaroslaw refuses, and Marek orders his guards to drag Jaroslaw off to an unknown fate.
The next day, as Connor was walking down the hallway with his best friend, Neil Radzewicz, and Neil's girlfriend, Sarah Weitzel, the three of them are confronted by Janine Lemieux, a girl from Quebec and the head of the school's cheerleading squad, who verbally abuses Sarah, and then Neil when he defended Sarah. After Connor steps into the fray, Janine dismisses him, saying "you don't even have a girl to call your own". That night, as Connor's parents are heading off to see the Cubs play in their first game of the Major League Baseball playoffs, Connor asks Kasia if she wants to get some ice cream. Connor introduces Kasia to the concept of a car, which she becomes freaked out over, and the concept of a train (again, she gets freaked out over this too). At the ice cream shop, Connor meets his cousin, Albin Pytleski, and after he and Kasia finish their ice cream, heads home with Kasia to introduce her to baseball - the Cubs win, and are a step closer towards the championship.
Two weeks later, Kasia, after using the basement bathroom in the early hours, returns to the futon she was sleeping on, carelessly forgetting to close the door behind her. The next morning, Kasia finds the Pytleskis' cat on top of her, and screams. The scream attracts the attention of Connor's parents, who find Kasia and wake up Connor, demanding to know why a girl was living in their basement. Kasia then explains who she is, and Connor's parents let her continue staying with them, making the rationale that they learned how much she and Emily get along with one another.
That night, the Cubs reach their first World Series in 71 years. Connor, a Cubs fan the whole 18 years he had lived, had awaited the day he would see the Cubs win the pennant, but his 81-year-old grandfather, George, reminds him about the Curse of the Billy Goat. Sure enough, the following week, the Cubs are down three games to one, and Connor grows skeptical of the Cubs' chances, but Peter convinces Connor that the series is far from over.
Two days later, it's Halloween. Kasia, having never trick-or-treated in her life, goes trick-or-treating with Peter and Emily while Connor and Albin watch a movie with Connor's friends. Two days after that, the Cubs win the World Series, and Connor's family goes up to Wisconsin to visit the grave of Connor's great-grandfather Charles and great-grandmother Margaret, both of whom had died without seeing the Cubs win a World Series.
Later in November, at Thanksgiving, Kasia still hasn't made much progress getting used to modern life, as evidenced by the fact that she's still used to lavish banquets for meals. In December, Connor decides to introduce Kasia to his friends. The next day at school, Michael Zemaitis, the quarterback of the football team and Janine's boyfriend, confronts Connor in the lunch line, saying that Connor's name might be on the ballot for prom king that year. Connor matter-of-factly tells Michael that he's not going to prom because he doesn't have a date, and Michael teases him for this. That afternoon, some of Connor's friends meet Kasia. One of his friends, a Sikh named Dalraj, asks if Kasia normally wears her dress from the past, and Connor says "Just for special occasions". Another of Connor's friends, a Japanese girl named Masami, asks "Like prom?" and Connor says "Sure, why not?"
That night, Connor has a bizarre dream, a dream that Kasia also has. The following morning, Kasia asks Connor what prom is, and Connor explains it's a dance. Kasia enthusiastically offers to go with Connor, even though it's months away. Unfortunately, in the days that follow, Kasia exposes to Connor how she's still homesick for her old life.
On Valentine's Day, after seeing Emily and a friend of hers from school named Henry hugging one another, Kasia confesses her love for Connor to him.
After a spring break with Connor's aunt and uncle in San Diego, wherein Connor realizes Kasia is truly starting to get used to modern life, it's time for prom. Connor gives Michael the surprise of his life when Kasia enters the ballroom. Later, Michael realizes that something will go wrong, as he did not expect Connor to show up with a date. Janine realizes this as well, and they have a change of plans. When it comes time to announce prom king and queen, Janine wins prom queen, and in a shocker, Connor wins prom king. Michael and Janine reveal that the ballot had been stuffed in favor of Connor as part of a prank to humiliate him, but they had come to the realization that they had mistreated Connor all throughout high school and, having learned that Connor had spent time getting a refugee from another time period used to modern life, realized that Connor was not the loser they thought he was.
A week later, Michael still feels bad, so he writes a note which his friend Kyung sends to Connor. The note tells Connor to come to Michael's house the next day with Kasia, and Michael reveals to Connor that he's an anime fan, making him and Connor not very different from one another. Connor is just bewildered at this revelation. Unfortunately, after getting that note, Connor's friends are led to believe that he no longer needs them, but Connor decides to meet his friends for lunch one Saturday to show that he's still loyal to them.
The next month, Connor has finished high school, and is given a standing ovation at graduation. Two days later, the Pytleskis are heading to Orlando for their vacation at what Connor describes as "the happiest place on Earth" (Walt Disney World, even though it's never referred to as such). Zoja is watching what's going on via her crystal ball, and Jaroslaw, alive albeit brainwashed, enters Zoja's house, demanding to know what happened to Kasia. Zoja shows him the crystal ball, and Jaroslaw threatens Zoja with violence, demanding her to hand over Kasia. Zoja, acting impulsively and fearfully, sends Jaroslaw off to the 21st century.
Emily mistakes a freshly-arrived Jaroslaw for veteran hockey player Jaromir Jagr. Jaroslaw, angered at being called "Jaromir Jagr" again, kidnaps Emily. Zoja (presumably) sees what's going on and sends herself to the future as well, still unsure about what to do, and meets up with Connor and Kasia, mentioning that Jaroslaw is brainwashed. Connor remarks that he doesn't remember his mother telling him that in the story - in his mother's version, Jaroslaw returns to Zoja and they live happily ever after. The words "happily ever after" trigger something in Zoja, who realizes how she can fix Jaroslaw: a true love's kiss. Connor, Kasia, and Zoja find and confront Jaroslaw, and after disarming him of his knife, Zoja delivers the kiss, and Jaroslaw is cured of his brainwashing and proceeds to untie Emily. Zoja makes the offer to Kasia to return home to the 16th century, but Kasia has now gotten used to modern life.
Jaroslaw and Zoja return home, without Kasia. After being told by Zoja that King Marek had brainwashed him, Jaroslaw takes Zoja with him to the castle, and find Crown Prince Karol, Kasia's older brother, being crowned king. Jaroslaw and Zoja then learn that King Marek had died of a heart attack while Jaroslaw was gone, and assure the royals that Kasia is happy where she is now. The next day, Jaroslaw and Zoja are married at last.
Back in Chicago on July 4, the Pytleskis and Kasia watch the fireworks. Connor remarks "So this holiday is called..." and Kasia immediately replies "Independence Day. I am now completely independent of my past life."
17 years later, on her 34th birthday, we see a pregnant Kasia finishing the story of how she met Connor to their young son and daughter.
