When she is a little girl Fiona's aunt Martha moves from Canada to Paris. Years later, Fiona receives a letter from her aunt Martha which asks her to come to Paris to help her since she is about to be forced into an old age home since she is 88. Fiona arrives in Paris only to find her aunt missing. Posing for a photo near the Eiffel tower she falls into the Seine and loses her backpack and belongings. The backpack is found by Dom who lives in a tent by the river. He uses Fiona's money to treat himself to dinner at an expensive restaurant where he runs into Fiona. The two dance and he offers to walk her home. While he is paying Fiona notices that he is wearing her purse and confronts him. He runs away from her and she falls in the Seine a second time. Dom realizes he is in love with Fiona and returns her backpack to the Canadian embassy where she retrieves it. She goes in search of her aunt, but is told she is dead and the funeral is happening that day. She goes to the funeral, Dom following behind. When they arrive they are asked to give a eulogy and Dom does, despite not knowing aunt Martha, at first saying generic pleasantries before giving an impassioned speech that Martha was a secret racist who hated badly dressed people as well as the homeless. During the reception Fiona realizes that she is at the funeral of Marthe, her aunt's friend. Going to the coffin to check on the body, Dom accidentally gets his tie caught in the coffin lid, and Fiona gets her nose stuck in the elevator trying to help him. When she is revived she is handed an urn containing the remains of Marthe, which she thinks also includes Dom's ashes. In reality Dom is fine, though when Fiona learns this she slaps him angrily. Meanwhile, Martha has been on the run from nurses from a retirement home that keep visiting her, hoping to persuade her to move in. She crosses paths with Dom several times and nearly crosses paths with Fiona. After Fiona slaps him Dom goes home to his tent and gets drunk, running into Martha who is hiding out in that location. He offers her some of his champagne. Later that night Fiona falls asleep in Martha's apartment, while Dom falls asleep in his tent with Martha. Dom and Fiona have an erotic dream about each other, leading Dom to have sex with Martha. Afterwards, Martha goes outside and retrieves Fiona's phone from a garbage bin. The two call each other and briefly connect, with Martha saying she has been drinking champagne in New York while sleeping with an attractive man. Fiona puts the clues together to find that Martha is on Île aux Cygnes near the replica of the Statue of Liberty. Arriving and seeing Dom, she realizes that he and her aunt had sex, but asks him to help her find her aunt. He has his dog pick up her scent which leads them to the Eiffel Tower. Fiona and Dom climb up the Eiffel Tower where they find Martha asleep in a satellite dish. The three pause and admire the view of the city before Martha dies. After her cremation a small ceremony with Fiona and Dom is held at Île aux Cygnes before they throw her ashes in the Seine. Fiona then reveals that she always intended to learn French and asks Dom if he will help her. He agrees.
On review aggregator website Rotten Tomatoes, the film holds an approval rating of 83% based on 47 reviews, and an average rating of 6.9/10. The website's critical consensus reads, "Lost in Paris is whimsical to a fault, but its fanciful light-heartedness earns the audience's indulgence with charming performances and an infectious absurdity." On Metacritic, the film has a weighted average score of 74 out of 100, based on 14 critics, indicating "generally favorable reviews".
