The movie starts out with two burglars rummaging through a desk, and we see Nancy Drew (Emma Roberts) hiding in a closet. She accidentally sneezes and is discovered. She talks to the burglars and reveals that she knows they are responsible for a chain of burglaries and that they are working with the locksmith. She also points out that the security guard would be walking into the room but if they cooperate, the DA will let them off easy because the police don’t want them. They start talking and one of the burglars agrees to make a deal with Nancy. She offers a lemon square because one of the burglars is cranky since he had not eaten. When walking out of the building the cranky burglar decides to back out on the deal but before they even realized it, Nancy Drew had already climbed out onto the roof to inform the police that are outside. Already familiar with the local law enforcements, Chief McGinnis (Cliff Bemis) isn’t surprised to see who solved the case.
Once the burglars are caught, we find Nancy still on the rooftop, but then she accidentally slips. With every adult below panicking to rescue her, she calmly grabs a rope and hooks it to the roof and easily propels down the side of the building. She is greeted by her father (Tate Donovan) who is also a detective but is worried for her safety. Here we also meet Nancy's two friends and one guy friend. We can tell that she likes him and he vice versa. We also see Nancy talking to the chief who tells her she is just like her mother, a topic that is sensitive for Nancy. They take photos for the newspaper - Nancy Drew has saved the day.
At home her father tells her that he wishes she would not sleuth anymore because it is dangerous, especially in the town where they will be moving into for his new job. While saying goodbye to Ned (Max Thieriot), the boy she likes, she tells him that she picked a house in Hollywood that had an old mystery. An unsolved mystery about a famous actress Dehlia Draycott (Laura Herring) who was murdered after she mysteriously vanished then reappeared. Leaving her small town she tells the Chief that she will no longer sleuth, and he replies back that he does not believe her.
Nancy and her dad take a train ride to Hollywood and see the old house for the first time. They meet Barbara who’s the person who sold them the house. Inside Nancy finds that the house is rigged with projectors that flash the image of the dead actress on the walls. At first Nancy thought it was a real ghost.
She also meets the grounds keeper, Leshing (Marshall Bell), who is quite eerily looking. At night, Nancy hears creaks in the attic and calls Leshing to make sure he was not in the house, but when she reaches the attic no one is in there. In the attic Nancy finds a book or journal that belonged to Dehlia, and inside she finds a note. The note addresses someone named Z and it tells Z that she must rewrite her will for she has someone else she must take care of, and it is signed D. Telling herself that she must not sleuth, Nancy tries to go sleep and get ready for her new school the next day.
At school we can see that the kids do not dress, behave, or have anything in common with Nancy. They dress more "rock and roll" and Nancy is more of an older but cuter style of her own. Nancy excels at everything but embarrasses herself in acting class when she performs the scene of Juliet killing herself when all the teacher had asked for was her name. At lunch we see all the other children eating junk food and being very messy, but then we see Nancy’s lunch, which are neatly cut finger sandwiches with vegetables and a cupcake on the side. Two girls, Inga and Trish, text each other one saying "I'm sitting next to Martha Stewart" and Inga tells Trish that she will distract Nancy and Trish should steal her cupcake. Inga talks and Trish steals, but they are spotted by a twelve year old, Corky (Josh Flitter) who tries to stop them but couldn’t.
At an assembly we see the entire school gathered for a basketball game, but suddenly Corky starts to choke and Inga yells for anyone who can do CPR, which of course Nancy responds to. After successfully dislodging the food, Corky still isn’t conscious so Nancy is about to give mouth to mouth when all of a sudden Corky laughs. Inga yells at him that he did it wrong, that he was suppose to kiss her to embarrass Nancy. We see that Inga is Corky's older sister. With her feelings hurt Nancy goes home and decides to stick to things that she loves like sleuthing.
At home, Nancy opens the door to find Corky there to apologize for what happened at school, he has a crush on Nancy. She lets Corky in and tells him that she has just rearranged the shots of the last movie Dehlia was in, in chronological order and noticed that her last scenes were all shot of her from the waist up, which leads Nancy to think that she as pregnant at the time. She goes to the hospital to receive birth records but because they were confidential, Nancy has to bribe the guy with another baked good, which lead to her receiving the information that newborns were recommended to have a pediatrician and that that information was not confidential.
At the other office we see tons of different folders and Nancy asks the lady how many births were there on a certain date and the lady finds the file surprisingly quick and tells her there were three births. She says she does not know if she is allowed to tell Nancy anymore and wants to call the office. The phone rings and she is distracted long enough for Nancy to copy down the three names. When the lady is done she looks at Nancy and knew she was trying to peek. She says nice try squirt not realizing Nancy was already done peeking. Nancy also talks to Mr. Biedermeyer who used to manage Dehlia, but being a busy man he walked away without saying too much.
Nancy searches for all the possible listings on the internet and goes on her huge search. After receiving many many no's she finally found Jane Brighton (Rachael Leigh Cook), who was adopted and tells her that her mother was the late actress Dehlia Draycott and that if they can prove they are related and find the will Jane can get help with her financial situation. Jane is happy to hear that she and her daughter may not have to live in their little apartment anymore and is willing to help Nancy with any questions. At this time Nancy is receiving threatening phone calls saying she needs to get off the case or else she will get hurt. Not caring or telling her father Nancy continues, but when she tries to reach Jane again, Jane seems scared and tells Nancy to go away. Clearly someone has threatened her and her daughter.
Another night, Nancy believes she is dreaming that she sees Dehlia telling her that the will is in the Chinese box. Nancy asks Leshing about the china box and he tells her that it was all sold. And Nancy sees a store circled in the yellow pages and figures that's where the box must be. It is almost Nancy's birthday and her dad calls her telling her that for her being so good about not sleuthing he got her a present and its at the front of the house. The doorbell rings and Nancy opens it to see her blue roadster and asks her dad how did he get it here when Ned was out and she realizes that he drove it here. Catching up all of a sudden Corky comes and tries to interrupt Nancy and Ned. Inga and Trish spot Ned and have crushes on him so they decide to follow them to their lunch date. They decide to eat Chinese but truly Nancy wants to go to the shop that the Chinese box was sold to. Inga and Trish follow them and Nancy calls everyone to search through all the boxes and that they are looking for a will. The search is unsuccessful. So Nancy decides to go back to her house and explore the attic.
In the attic, Nancy, Ned and Corky find a secret door. Nancy finds out that was how the mysterious intruder has been getting in and out unnoticed. She decides to explore the passageway and finds it can open a secret place behind a fireplace and if she crawled through the tunnel it ends up in an old ladies laundry room. Surprised, Nancy suspects Leshing of being Z and that he must know of these tunnels and that’s how he gets from one place to the other so quickly.
Back at school, Nancy passes out invitations to her birthday party but no one seems to want to go. Inga tells Trish that they must go because Ned will be there. That night Inga and Trish arrive and bring in dozens upon dozens of party guests. Then Trish is found choking and faints to the floor. At first Nancy thought it was another trick but soon realizes that Trish really needed help. Taking emergency precautions Nancy orders a knife and a ball point pen and does something that makes everyone queasy. The ambulance arrives and Inga tells Nancy that she is actually a pretty cool person and that she and Ned should come shopping with her tomorrow.
After everyone has left, Nancy’s father arrives and the police tell him that there were complaints by the neighbors of loud noise. Surprisingly, Nancy’s father is glad she had a wild party because that is what un-sleuthing teenagers do.
At the mall, Inga wants to have the lady working in the shop critique what Nancy is wearing but instead she critiques what Inga is wearing and actually loves what Nancy has made and decides to take a picture of it.
The next day, Nancy finds another present from her father, a brand new cell phone. The doorbell rings and Jane arrives at Nancy’s crying, saying that social services has taken away her daughter because they claim she was an unfit mother. Jane tells Nancy that a man with one light blue eye and one dark blue eye came and threatened her and her daughter. Nancy calms her down and watches a TV biography of Dehlia with Jane and a scene from one of the Dehlia's movie is played. The scene is of Dehlia searching for a will and an old Chinese man saying that to find the will you must make the dragon bow. She rotates the dragon, which reveals a hidden button. Pushing the button she finds a secret compartment where the will is hidden. Nancy realizes that her dream was probably another one of the projectors rigged in the house but also that Dehlia must have used the same box to hide her will in. Nancy returns to the Chinese shop and indeed finds the will. The shop owner decides to give her the will. On her way out Nancy calls her dad who asks what phone is she calling him from and she says the new phone that he gave her. But her dad says that he did not give her a new phone and suddenly Nancy sees her call is being traced but it is too late as a van pulls up and two guys come and grab Nancy.
Waking up, Nancy finds herself in a room full of movie projectors and sees the henchmen below watching movies and sleeping. Sneaking out the window and onto the scaffolding above she tiptoes across the ceiling and even fishes out the will from the desk. But then, lightbluedarkblue man tells another goon to check on Nancy. When they realize she’s gone Nancy dashes for the door and jumps on a truck to escape. She finds her car but is chased by the henchmen. They rear end her and she swerves out of control and ends up in the police station and her dad picks her up. With her dad is also Mr. Briedermeyer and he offers a ride back with him.
In the limo, Nancy and Mr. B talk about the will and Mr. B asks to see it. Mr. B signs a document and his signature is just a big Z. He says that his middle name is Zachary and Nancy realizes that he is Z and sees that the driver is actually lightbluedarkblue eye man. She jumps out the car but her dad isn’t able to and is punched in the face. Running for her life she arrives at the old ladies house again and find out that Mr. B owns the entire property. Running to the laundry room Nancy crawls through the tunnel only to run into Mr. B who confesses to her why he killed Dehlia. She was his first and at that time only client and after having Jane, Dehlia wanted to quit acting. In a rage he killed Dehlia and the father of the baby is Leshing.
Nancy makes a run for it and the henchmen and Mr. B end up chasing her up to the room where the fireplace is. When all seemed lost Leshing appears behind the fireplace and hits Mr. B with a shovel. Not knowing what had just happened Inga and Trish walk in telling Nancy how they love the new penny loafer trend and that Nancy has appeared in a fashion magazine. The police arrive and arrest Mr. B and Nancy tells Leshing that he is Jane’s father and the long lost family is able to reunite.
We see Social Services letting Jane's daughter return to her family and seeing her grandpa for the first time. Jane returns home since Mr. B was her dad’s employer.
Nancy talks to Ned and they finally share a kiss. He says that she has depression that finishing the case made her sad and that she was happy with trouble. Then her father shouts to her there was a new case in Scotland about the Loch Ness Monster and diamonds. With another case to solve Nancy Drew keeps on saving the day.