This film is the story of a young woman's rise to success and the unconventional road she took to get there. Tess McGill (Melanie Griffith) is a secretary trying to get ahead at her Manhattan financial conglomerate. She is 30-years-old and earned her college business degree at night school. She rides the Staten Island Ferry to work every day which is a large corporation called Petty Marsh. Her goal is to move out of the secretarial pool and into an executive position. Yet, her dream remains unrealized; because her education was not acquired at a prestigious school, she cannot gain entry to her firm's training program for recent college graduates. She reads everything she can get her hands on, and endures the too-often falsely dangled opportunities of her superiors.
After being misled into thinking that her supervisor, David Lutz (Oliver Platt), is helping her get a better job, Tess finds herself set up with a lascivious and debauched colleague of Lutz, named Bob Speck (Kevin Spacey) who tries to seduce her in his private limousine. She is forced to run out of the limo onto the street to get away from Speck. An angry Tess returns to the office and then gets into trouble by publicly calling her boss a "sleazoid pimp with a tiny little dick" on the office's digital stock-ticker crawl display.
Following this, Tess is reassigned as a secretary to a new financial division for Petty Marsh and goes to work for company's new mergers executive, Katherine Parker (Sigourney Weaver), who is two weeks younger than she and the first woman she's ever worked for. Katherine assures her that their relationship is a two-way street and offers to mentor Tess. Seemingly supportive and benevolent, Katharine encourages Tess to share any good business ideas she has.
Tess is delighted to finally be getting somewhere much to the chagrin of her live-in boyfriend, Mick Dugan (Alec Baldwin), whose main ambition is to buy his own fishing boat and doesn't understand the time Tess spends in Speech and Management classes that she attends on the weekends since Tess still speaks with the voice of a 10-year-old girl. Tess presses on, despite Mick's indifference and the bewilderment of her best friend Cynthia (Joan Cusack), who is engaged herself, and doesn't understand why Tess doesn't just marry Mick. Tess tries to explain to Cynthia that her career comes first.
Over the next few days, Tess observes Katherine's style, ease with power, and smooth handling of the lecherous men who seem to populate their world. One day, Tess comes to Katherine with an idea for one of the firm's clients. Trask Industries has expressed interest in acquiring television stations, but Tess says that her research shows Trask would be better served to acquire radio instead. Katherine quizzes Tess on the genesis of this idea and, once she's convinced it really is Tess' own, sends her on her way with promises to bring it up at the next meeting. Tess is elated.
As Katherine prepares for a getaway ski weekend to Europe with her estranged boyfriend, she confides to Tess that she's sure this is the weekend he'll pop the question. Katherine learns that Jack can't join her skiing but decides she should have a good time anyway... and promptly breaks her leg while skiing down a slope. She calls Tess from her hospital bed with a flurry of instructions: answer invitations, inform people that she'll be incapacitated, and take care of things at Katherine's parents house in New York as well. She apologizes for leaving such a mess, but reassures Tess that she can "make it happen."
Tess arrives at Katherine's family's elegant Manhattan townhouse and soaks up the life she wants for herself. She sits at Katherine's make-up table, uses her exercise equipment, and imitates recordings of Katherine's upper-crust cadence. Then she finds a memo Katherine recorded for herself about Tess' idea for Trask Industries. She checks Katherine's home computer and, sure enough, finds more evidence that Katherine is intending to pass off Tess' idea as her own.
Tess returns home to Staten Island, crushed at Katherine's betrayal, only to find Mick engaging in a worse betrayal; having sex with another woman, whom is one of her friends named Doreen DiMucci (Elizabeth Whitcraft). Disillusioned with Katharine and disgusted with Mick (whom she calls a "snake"), Tess walks out into the night.
Having nowhere else to go, Tess runs back to Katherine's house to shack up there. Cynthia brings Tess her clothes from the apartment and begs her to talk to Mick, but Tess has decided to take action. She sees Katherine's invitation for a party that night from Dewey Stone & Company, which Katherine had intended to bring in on the Trask deal. In a bold move, Tess sets up a meeting for the following morning with Jack Trainer of Dewey Stone (the radio expert that Katherine had mentioned in her memo) and accepts the invitation to the party herself. She and Cyn go to Katherine's to borrow a $6,000 dress for the party; and, at Cyn's suggestion to calm herself, she takes one of Katherine's Valium. With Cyn's help, Tess remakes herself and attends the party looking different but still feeling slightly out of place.
Once at the Dewey Stone party, she realizes how remote are her chances of finding Jack Trainer (Harrison Ford), and getting a head start with him before their meeting has decided to leave; but a handsome stranger approaches and asks her to have a drink with him. Tess asks the stranger if he knows Jack Trainer; and, of course, the stranger is Jack Trainer, who conceals his identity from Tess. After a few drinks, Tess realizes how intoxicated she is and should leave and asks the stranger to please get her coat for her. He brings the coat to an incoherent Tess in a waiting cab and instructs the driver to take them to his place. Once there, he carries the now totally unconscious Tess up to his apartment and puts her to bed.
The next morning Tess awakens, thinking the worst, and moves as fast as she can to get out of the apartment before having to deal with the sleeping stranger beside her. She escapes with seconds to spare and regains her focus for her meeting at Dewey Stone. Draped in one of Katherine's stunning suits, she quickly realizes that the handsome stranger from the night before was Jack Trainer. He and his colleagues praise her idea for Trask's acquisition of a radio network but are curious why (1); she's come to them, and (2); has no briefcase. She gives them a plausible story on both counts, and they agree to meet further within their company and let her know if they can be part of the deal.
Back at Tess'--no, Katherine's--office, Tess is cursing her own bad judgment concerning men and alcohol; and Cynthia is cursing Tess' double life, warning that she'll be out of, not only her man and her home, but her job as well. Suddenly, Jack appears on the floor, asking to see Tess. Tess convinces Cynthia to pose as her secretary and meets with Jack. After Tess' questions about the previous night, Jack assures her that nothing untoward occurred and that Dewey Stone is ready to proceed on the Trask deal. He gives her a gift of a new briefcase to celebrate and invites her to dinner; but she declines in spite of the attraction growing between them.
Now the frenzy of activity kicks into high gear: Tess playing her role as Katherine's secretary for Katherine when she calls and for everyone else at her company and the role of deal manager with Jack Trainer. They work well together, making it fun and flirty when they can. When Tess attends Cynthia's engagement party, she realizes how much she's changed in a short time, how she no longer fits into the world she once called home. After Mick's impulsive marriage proposal, Tess says good-bye to him for good.
A few days later, Tess and Jack crash Oren Trask's (Philip Bosco) daughter's (Barbara Garrick) wedding, intending to make an appointment and pitch their plan. They succeed and meet with two of his executives. Trask is interested. After a successful meeting with Trask, Tess and Jack sleep together for the first time. The next morning, Jack confesses that he needs to break up with his girlfriend. For him it's already over, he just hasn't told her yet. Tess' disappointment is evident when she learns the mystery girlfriend is Katherine and decides to keep her identity as Katherine's secretary a secret.
Everything begins to come to a head: Katherine is returning to Manhattan the same day Tess and Jack are scheduled to finalize with the principles of the Trask deal, including the owner of the radio network that Trask is acquiring. Tess quickly cleans up and moves out of Katherine's house, dry cleans all Katherine's clothes, and meets her on the company's helipad when she arrives.
At Katherine's house, she wastes no time drafting Tess to help her unpack, bathe, and change into sexy lingerie so that she can reconnect with the man she is so sure will propose to her. As Tess is about to leave on an errand to get painkiller pills for Katherine, she mentions to Katherine the memo she found that started the whole chain of events and points out that it looks like she's proffering the Trask radio idea as her own. Katherine claims that it was a necessary measure, since once Jack was accused of stealing another's idea. Katherine calls Jack to come over and, after a brief struggle during which Katherine proposes to him, he leaves without telling her about Tess. Tess returns from the errand and hides in a coat closet when she sees Jack. After Jack leaves, Tess emerges from the closet, gives Katharine her pills and rushes out of the apartment, accidentally leaving her appointment book behind. Katharine reads it and becomes enraged after she discovers what Tess has been up to for the past few weeks.
Before entering the boardroom to finalize the Trask deal, Tess quickly asks Jack if he's ever had any ethical problems about looking over the work of others--he says he hasn't. The two confess their love for each other and sit down at the table. Suddenly, Katherine, on crutches, bursts into the room, screaming that Tess is an imposter and merely her secretary. Tess is unable to make the others understand that Katharine is the liar and leaves the meeting in shame. All the deal participants, including Jack, are confused.
A few days later, Tess attends Cynthia's wedding where she finds that Mick is going on with his life with his new girlfriend, Doreen. After the wedding, Tess returns to her office to clean out her desk where the rest of her co-workers wish her the best of luck.
In the lobby, she runs into Katherine, Jack, and Mr. Trask along with a group of executive businessmen. The women confront each other and get into a huge shouting mach in front of everyone. Katherine again play-acts as the "poor innocent victim" and accusing Tess of being a thief and liar (Note: the sociopath Katherine is basically describing herself by passing everything onto Tess using the old fashion 'accuse-your-accuser' manipulation tactic), while Tess accuses Katherine of pretending to be her friend so she could steal her ideas... as well as ploting to fire her on trumped-up charges after the deal goes through. When Jack intervenes in the argument, Tess tells him that her impersonating an executive may have stared out that way, but everything else she told him, as well as their night together, was true and that Katherine was always the bad person hiding behind her charming facade. Katherine whispers to Tess at what she told her earlier that "it was all just business". As the crafty and manipulative Katharine tries to lead the group onto the elevator, Jack then refuses to come, stating that Tess is this team's leader and he believes her. Trask tells Jack: "you better not let your Johnson go on making business decisions for you."
Katherine cautions Mr. Trask to ignore Tess and not to belive a word she say. Suddenly remembering a little tidbit about the Trask merger, Tess asks Trask if Katherine had told him about the potential dealbreaker. Tess explains about a conflict of intrest with a fellow Trask Industries client that could ruin the merger if it is to be made public within a week after the deal closes. Trask is stunned to hear Tess explain the potential loophole in the deal and realizes she may be telling the truth. Jack maneuvers himself, Tess, and Mr. Trask into an elevator alone; and Tess explains how she first conceived the idea for Trask to acquire radio and shows him the newspaper clippings that inspired her.
When the elevator door opens again on the conference floor, Trask approaches Katherine and asks her where SHE got the idea for the deal? Katherine pretends not to remember and says that she needs to check her files in her office, but Trask refuses to let her go to her office and he again asks Katherine where she came up with the idea for the Trask Industries radio deal merger. Katherine tries changing the subject, and then says that it's complicated, but Trask tells her to uncomplicate it. When it quickly becomes clear to everybody present that Katherine cannot (and will not) provide an answer, Trask says he will insist to the Petty Marsh board of directors that she be fired for her deception. He then offers Tess a job at his firm if she promises to bring the passion and integrity she brought to his deal.
A few days later, Tess moves in with Jack at his apartment and starts her first day of her new job at Trask Industries. When she arrives, she sees a woman in an office, talking on the phone, and presumes she is to be the woman's secretary. However, it is quickly revealed that the woman is, in fact, Tess new personal secretary Alice (Amy Aquino), and Tess realizes that Trask's definition of "entry-level" involves the opportunities she had dreamed of. She insists her new secretary treat her as a colleague, rather than a superior, proving she is going be a very different boss from Katharine. Tess then calls Cynthia at her secretary job across town, to tell her the good news by saying: "Guess where I am?" Cynthia shouts to everyone that Tess has finally made it out of the typing pool.