Chloe (voice of DREW BARRYMORE) is a pampered Chihuahua that lives a highly pampered and high society life in Beverly Hills with her human owner, cosmetics tycoon Vivian Ashe (JAMIE LEE CURTIS) who loves her dog more than anything else in the world. Accordingly, when she has to leave on a multi-city business tour, she asks her young adult niece, Rachel (PIPER PERABO), to watch Chloe. Rachel agrees, but then heads south of the border to Mexico with her friends. While out partying, Chloe manages to escape from her hotel room and is then dog-napped by men working for dogfight ring leader, Vasquez (JOSÉ MARÍA YAZPIK).
There, little Chloe is pitted against the menacing Doberman pincher champion, Diablo (voice of EDWARD JAMES OLMOS), but is saved at the last minute by Delgado (voice of ANDY GARCIA), a world-weary but still capable German shepherd with a troubled past. Having done his duty, he wants nothing more to do with the spoiled pooch, but feels sorry for her and reluctantly agrees to help try get her back home. At the same time, once Vivian's landscaper, Sam Cortez (MANOLO CARDONA), and his dog, Papi (voice of GEORGE LOPEZ), who's in love with Chloe, hear of her disappearance, they set out on a rescue mission, eventually joining Rachel.
As they try to find the missing dog, Delgado tries to get her to safety, all while having to deal with the conniving rat Manuel (voice of CHEECH MARIN) and his iguana friend Chico (voice of PAUL RODRIGUEZ) who are after Chloe's diamond necklace, as well as Diablo who's hot on their trail and determined to bring the valuable dog back to his master.