The movie starts out with some guys robbing a jewelry store. The robbers rob the store and are getting into their getaway car when all of a sudden you see a flash of light go through the sky; it's G-girl (Uma Thurman). G-girl then lifts the car off the ground with the robbers in it and drops the car right in front of the police station telling the police men that they can now take it from there. From what we see we now know that G-girl is the local super hero.
We then see a man sitting in a jail cell with many posters/articles of G-girl posted on the walls. This man is Professor Bedlam (Eddie Izzard) and he is G-girls' arch enemy. Professor Bedlam promises to seek vengeance on G-girl and escapes from his jail cell by electrocuting the guards.
Next see Matt (Luke Wilson) and his friend Vaughn (Rainn Wilson) talking about G-girl and how hot she is while riding the subway. Matt expresses how much he misses his ex-girlfriend and Vaughn quickly tells him to get over her because it's been about 8 months and Matt tells him that it's too soon to start dating again, he knows, but he really digs Hannah (Anna Farris), the girl he works with. Vaughn reminds him that Anna's taken and points out a girl sitting alone across from them and tells Matt to go over and talk to her. Matt gets the courage and goes up to her, she introduces herself as Jenny Johnson (we clearly see that she is G-girl but with brown hair instead of blonde) and basically blows Matt off when he asks her on a date. As the subway comes to a stop a purse snatcher runs off with Jenny's purse and Matt runs after him. After some running and some humorous lines Matt gets Jenny's purse back and hides in a dumpster in order to hide from the purse snatcher. Jenny finds Matt in the dumpster and thanks him for getting her purse back and says that no one else has ever saved her before, she then accepts to go out on a date with him. As they walk away we clearly see the purse snatcher hanging from the ledge of a building where Jenny/G-girl put him.
For their first date they go out to a fancy restaurant and we see that Jenny is very talkative, nervous and bit neurotic. Jenny has super hero hearing so she hears that there is fire somewhere and she excuses herself in order to go the ladies room. She then flies off to the location of the fire and puts it out then flies back to the restaurant, obviously exhausted. Matt notices but says nothing because he already thinks she's a little strange.
Next we see Matt and Vaughn at the gym, Matt is telling his friend about the date and how strange she seems to act but he thinks he still might go out with her again. Meanwhile Vaughn isn't even trying to listen to Matt because he's too busy checking out a girl that's working out next to them (it's pretty clear that Vaughn loves the ladies but he never gets lucky with any of them). Finally Vaughn tells Matt to just get laid and then leave her and Matt asks himself why he even bothers to talk to him when he's no help.
From what we know Matt works for an architectural firm and is co-workers with Hannah (Anna Farris) who clearly has no clue that Matt is into her because she is infatuated with her male model boyfriend. Matt and Hannah's boss is Wanda Sykes and she does a bit of comic relief when she thinks that Matt is sexually harassing Hannah because of a scene where Hannah's on a stepping stool and Matt is checking out her butt and Wanda Sykes witnesses this.
Matt and Jenny go on a couple more dates and we find out that Jenny works for an art gallery. We also see that Jenny is getting more and more into Matt because she's becoming really aggressive and demanding with him. At the end of one of their dates they end up at Matt's apartment and things get hot and heavy when Jenny asks if she can be on top, she promises it'll worth it and we then see Matt's bed banging against the wall, breaking through the wall and the bed posts breaking as Jenny has the ride of her life. In the meanwhile Matt looks scared as hell and confused as to what is going on because his bed and wall are now in pieces. The next morning we see Matt walking down the street in obvious pain from the night before. At work Matt talks to Hannah about Jenny and their relationship and she advises him not to stick around if he's not into Jenny.
Matt and Jenny go out on another date and Jenny takes him to her apartment and says that she has something to show him. Jenny walks out of her room into the living room in her G-girl outfit. Matt thinks it's great that she wants to role play so he starts to undress when she stops him and she then takes off the brown haired wig unveiling her blonde hair. Meanwhile Matt thinks it's even greater that she got a wig to go with the outfit when Jenny tells him that it isn't a costume, that she is really G-girl. Matt is completely dumbfounded by this revelation but is also really excited because he's been doing it with G-girl and now he can rub it in Vaughn's face. Jenny tells him that he has to promise to never tell anyone about this or she'll shove a chainsaw up his butt, he promises and Jenny tells Matt about how she became G-girl. It all started in high school, she and Professor Bedlam where best friends because they were both geeks and one night when things were getting hot and heavy between them a meteor struck and she followed it while he stayed back. Prof. Bedlam then found her standing next to a giant neon colored ball. Jenny reached out to touch it and as soon as she did it sent her flying backwards into the ground. Within a matter of seconds Prof. Bedlam watched as Jenny transformed from a geeky brown haired girl into a voluptuous beautiful blonde. After that night nothing was the same for both of them, they both went they're separate ways and now he was her arch enemy.
Matt is in aw of the story and Jenny tells him that there is something she's always wanted to do but never has been able to until now. We then see Jenny taking Matt out flying, getting kinky and then doing it in mid air all while Matt's looking scared and ready to cry. The next day Matt talks to Hannah about Jenny and where their relationship is headed when Jenny shows up out of nowhere, obviously angry that Matt is talking to Hannah, when Hannah introduces herself and Jenny entirely ignores her. Hannah blows it off and Matt says that they should all go to dinner together sometime. Hannah agrees and they set a date for all of them, including Hannah's boyfriend, to go out to dinner. The night of the dinner date we see that Hannah's boyfriend was unable to make it and Jenny is very upset and annoyed that Hannah and Matt are so chummy. While at dinner the television announces that there is a missile headed straight for the city and it's too late for evacuation, all hope is lost. Matt looks at Jenny and pleads with her through looks to go and do something all while Jenny ignores all his gestures because she doesn't want to leave Matt and Hannah alone together. Everyone is freaking out around them and finally Jenny gets fed up and excuses herself. G-girl gets to the missile before it hits the city and pushes it off course back into space. The day is saved. After returning to the restaurant she and Matt leave and while in his car she tells him that she didn't appreciate him groveling all over Hannah during dinner. Matt tells her she's over reacting and Jenny turns around and punches his side window, breaking it and saying she's not over reacting. At this point it's pretty obvious that Matt is a little scared of her now. Jenny apologizes and asks if he's coming up to her apartment but he declines.
The next morning he tells Vaughn about the incident and decides to break it off with Jenny the next nigh at dinner.
The next night while Jenny is making dinner Matt decides to tell her that maybe they should take a break and maybe even see other people. Jenny completely freaks out, throws Matt into his refrigerator, throws and breaks things and tells him that he will regret breaking her heart. Jenny exits through his ceiling leaving a huge gaping hole in his roof as she flies off.
The next couple of days we see that Matt is obviously nervous, scared and on edge because he's afraid of what Jenny will do for revenge. While at work Hannah notices that Matt is acting strange and he tells her that he and Jenny broke up. Hannah then tells Matt the she too is no longer with her boyfriend, he was cheating on her. All the while that Hannah's talking to Matt, Matt notices Jenny outside a window (disheveled and pissed) watching both of them while they talk and instantly turns scared. We see Jenny take off and Matt get sent for. In the parking garage they have noticed that his car is missing, except that it went missing through a huge hole in the roof. Matt knows exactly what happened, Jenny took Matt's car and flung it into outer space where he later sees through a telescope that Jenny wrote “you suck” all over it while it floats in outer space.
Matt has had about enough of Jenny's antics when Professor Bedlam contacts him and tells him about a plan to retract G-girls powers. They're at Prof. Bedlam's house and Matt wonders into a room dedicated to G-girl/Jenny, it has all sorts of pictures of her when she was younger and present pictures and articles too. Prof. Bedlam's bodyguards get him out the room and Bedlam tells Matt that the only thing is that he cannot get close enough to her for his plan to work, he needs Matt in order to get his revenge. Matt tells Prof. Bedlam that he cannot help him and leaves.
Back at his apartment we see Matt is packing because he's decided to leave town when Hannah calls him. He doesn't pick up the phone in time and is worried about her safety so he runs to her apartment to see her. When he gets there Hannah turns out to be all right, she's just depressed about her boyfriend cheating. Matt tells Hannah that he's tired of being just her friend, that he really cares for her and they kiss and spend the night together. The next morning Jenny sees them in bed together and freaks out so she throws a shark into Hannah's apartment. Freaking out, both Hannah and Matt fight off the shark and it jumps out the window trying to eat Matt. They're both okay but Hannah's wondering why G-girl just threw a giant shark at them both. Matt tells her he has something important to do and he'll call her later. He leaves and calls Prof. Bedlam and tells him that he will help him with his plan.
Matt goes to Jenny's work with flowers and apologizes to her and she accepts and Matt tells her to meet him at his apartment that night for something special. Jenny shows up at Matt's apartment, the balcony is decorated with candles and it's really romantic. Jenny notices a huge present on the table and goes to open it but Matt stops her and tells her it's a surprise for later. They sit and talk and Jenny is so happy that he finally realized she was the one for him and not Hannah. Then we see Hannah walking into Matt's apartment and sees Jenny on Matt's lap and demands to know what's going on. Jenny tells her to get lost because he's with her again. Hannah looking hurts demands an explanation when Prof. Bedlam comes out and tells Jenny/G-girl that her time is up.
Jenny tells Bedlam that he's crazy and Bedlam opens up the huge present, inside is the huge meteor that once gave Jenny her powers. Jenny falls to the floor, turning back into her old plain self she looks at Matt and asks him why he did this. Hannah goes over to the huge meteor to touch it as does Jenny to the best of her ability and then suddenly there is a huge explosion and both go flying in different directions. Jenny gets up, obviously still G-girl, when we see Hannah also rise up into the air, only that Hannah now has red hair and obviously super powers as well. Both girls get into the huge fight going from land to air to outer space eventually ending up in the middle of a fashion show. Matt yells at both of them to stop and tells Jenny to stop because she's supposed to be the good girl and to realize that she wasn't really in love with him she was just infatuated with him. Matt goes on to tell her that there is however someone that truly loves her and always has, Prof. Bedlam. Matt tells Jenny about the shrine Prof. Bedlam keeps of her and that he's always been in love with her. Prof. Bedlam tells her it's true and they kiss.
In the end we see that Hannah has super hero powers like G-girl and she and Matt have stayed together. We also see that Jenny still has her powers as well and she and Prof. Bedlam have stayed together.
Jenny and Bedlam go over to Matt and Hannah's apartment because Jenny is returning Matt's car from outer space (the car is completely trashed) and apologizes for everything she did. Hannah and Jenny both hear (with they're super hero hearing power) that there is problem somewhere in the world and both go off to save the day, as partners.