Elle Woods (Reese Witherspoon) wants her Chihuahua, Bruiser, to reunite with his mother, because she would like Bruiser's mom to attend Elle and Emmett's wedding. Elle hires a detective to find Bruiser's mother, only to discover that the owner of her dog's mother is a cosmetics company that uses Bruiser's mother for "testing". She finds out that her law firm represents the C'est Magnifique Corporation.
Elle decides to leave Boston, where she and Bruiser have settled with her fiancé Emmett, and go to Washington, D.C. to work on Bruiser's Bill. Elle is upset that her dog's mother is in a make-up testing laboratory, and decides to take it upon herself to be the "voice for those who can't speak" and to outlaw animal testing.
While working for Congresswoman Victoria Rudd (Sally Field), Elle is met with skepticism and other barriers common to Washington politics. Rudd's member of staff, Timothy, sarcastically calls her "Capitol Barbie". (There has even been a Barbie doll based on Elle Woods.) After a variety of ups and downs including a failed attempt to improve her work environment by having her co-workers write compliments about one another and place them in the "snap cup", Elle starts to lose her faith in Washington politics.
As the story moves along, Elle discovers that Bruiser is actually gay, after she is paged by "The Paws that Refreshes: A Doggy Day Spa." Bruiser has been affectionate with Leslie, a Rottweiller owned by Congressman Stan Marks (Bruce McGill), the Chairman of the Committee on Energy and Commerce which has jurisdiction over Bruiser's Bill. Elle also finds that Congresswoman Libby Hauser (Dana Ivey), the Ranking Member of the same committee, was a member of Delta Nu (the sorority from the first film). As a result, both Marks and Hauser warm to Elle and eventually come to support Bruiser's Bill.
Elle also discovers that Congresswoman Rudd has actually been working against her. Rudd has been doing so in an effort to satisfy the interests of a major campaign donor named "Bob" (who is never seen, but with whom Rudd has several telephone conversations). However, Rudd is eventually blackmailed into supporting Elle's petition, because Rudd's Chief of Staff, Grace Rossiter (Regina King) eavesdrops on a recorded conversation during which Rudd admits to Elle that she has been working against Bruiser's Bill in order to help Rudd's sponsors who want to continue with tests on animals. As Grace is appalled that Rudd lied to Elle and blamed it on her, Grace and Elle eventually reach a place of mutual respect, especially after Grace admits she came to Washington D.C. with an enthusiasm not unlike Elle's, but later lost that idealism when she discovered how dirty politics could really be.
With the help of her friends, Elle's discharge petition is successful, and Bruiser's Bill is brought to the floor of the House. Elle and Emmett get married in a park in D.C., albeit not at Fenway Park as they had planned, but standing on the home plate which has been delivered to D.C. by the UPS Guy (Bruce Thomas). In the final scene of the movie, when Emmett asks where to live, Elle says, "Oh, I think I know just the place," as they are driving by the White House.