Tess McGill (Melanie Griffith) is an ambitious, working class 30-year-old from Staten Island with a bachelor's degree in business that she achieved by taking evening classes. She works as a stockbroker's secretary in lower Manhattan, aspiring to reach an executive position. Tess is treated like a bimbo by her boss and male co-workers, who nonetheless benefit from her intelligence and business instincts. Fed up with being humiliated by her boss, Tess quits in dramatic fashion.
Tess soon finds a job as an administrative assistant to Katharine Parker (Sigourney Weaver), a young associate in Mergers and Acquisitions. Seemingly supportive as fellow female professional, Katharine encourages Tess to share ideas. Tess suggests an idea for a merger between Trask Industries and a radio station. Katharine seems intrigued but eventually tells Tess it wouldn't work out.
When Katharine injures her leg skiing, she asks Tess to house-sit. While staying there, Tess discovers some meeting notes and realizes Katharine plans to pass off the merger idea as her own.
Tess decides to use her boss' absence, connections, and clothes to move ahead with her merger plans. She schedules a meeting with Jack Trainer (Harrison Ford), a mergers and acquisitions associate from another company. With her friend Cyn's (Joan Cusack) help, she cuts her hair to look more professional and raids Katherine's closet for more stylish clothing.
At the meeting with Trainer and his associates, Tess lacks confidence and leaves believing the meeting was a failure. Jack, however, arrives at her office and wants to move forward with her idea. Jack quickly secures a great radio network acquisition for Trask Industries and bristles when Tess attempts to meet with the Trask CEO, Oren Trask (Philip Bosco) on her own, which he soon realizes is because her plan is to meet with him while crashing his daughter's wedding. Despite Jack's misgivings, Tess's charm and quick thinking secures Trask's interest in the merger.
Jack and Tess grow closer as they prepare the financials for the merger proposal, which is ultimately a success. They give into their attraction and end up in bed. Tess is tempted to tell him the truth, but demurs when she discovers Jack is also involved with Katharine, though he was going to break up with her before her injury.
Katharine returns home the same day as the meeting to finalize the merger. While Tess is helping her get settled, Jack arrives to end things with Katharine, who pressures him to propose. He dodges the conversation and then runs to the merger meeting. Tess accidentally leaves her appointment book in Katharine's apartment before leaving for the same meeting, which leads to Katharine discovering what Tess has been up to.
Katharine pushes her way into the meeting and outs Tess as her secretary, accusing her of having stolen the idea. Tess begins to protest but feels nobody would believe her. She leaves, apologizing profusely.
Days later, Tess clears out her desk and then bumps into Jack, Katharine, and Trask at the lobby elevators. A confrontation between Katharine and Tess leads Jack to stand up for Tess. When Tess reveals she's discovered a hole in the deal, Trask abandons Katharine in a closing elevator and hears Tess's explanation for how she came up with the merger idea.
When Trask confronts Katharine, she is unable to explain where she got the merger idea. He promises to have her fired for her actions, then offers Tess an entry-level job with Trask Industries, which she happily accepts.
Tess arrives for her first day at her new job at Trask and is shown to an office where she meets Alice, the woman Tess assumes she will be working for, however Alice explains that she is actually Tess's secretary. Tess insists they work together as colleagues, showing she will be very different from Katharine. She then calls Cyn from her own office to tell her she has finally made it.
Melanie Griffith as Tess McGill
Harrison Ford as Jack Trainer
Sigourney Weaver as Katharine Parker
Alec Baldwin as Mick Dugan
Joan Cusack as Cynthia
Philip Bosco as Oren Trask
Zach Grenier as Jim
Nora Dunn as Ginny, a colleague of Katharine's
Oliver Platt as Lutz, Tess' sleazoid first boss
James Lally as Turkel
Kevin Spacey as Bob Speck, a cokehead arbitrageur
Robert Easton as Armbrister
Amy Aquino as Alice Baxter, Tess' Secretary
Olympia Dukakis as personnel director