Turner and Quaid play Jane and Jefferson Blue, a wise-cracking couple of spies for an unnamed U.S. covert organization on maternity leave in New Orleans with their baby daughter whom they dote on, though they are unable to agree on whether her name should be Louise Jane or Jane Louise. With the baby's arrival, they have decided to move on to "Chapter Two" of their marriage, retiring from field assignment in an attempt to give their daughter a normal life. However, events continually conspire to draw them back into their old lives, including fruitless attacks by a frustrated mugger (played by Stanley Tucci) while being called back into service to combat a psychotic Czech arms dealer (played by Fiona Shaw).
Jane Blue (Kathleen Turner) is devoted to her husband Jeff, their daughter Louise Jane (Jane is uncomfortable with naming her daughter after herself), and starting the new chapter in their lives. She can be quite charming or very mercenary, depending on what is called for in a situation. It is her call to take on the assignment involving Novachek, and only after negotiating a six-month leave with bonus of 50% annual pay to put toward Louise Jane's college fund. Though Jane is the more sensible one, she is always ready to back up Jeff in wisecracks or fights (He considers her the more dangerous in hand-to-hand combat). She is fluent in several languages, adept at picking locks, and expert in firearms and martial arts.
Jefferson "Jeff" Blue (Dennis Quaid) is devoted to his wife Jane, their daughter Jane Louise, and starting the new chapter in their lives—sort of. Having been thrown out of the FBI twice, and the CIA once, Jeff can't help teasing and trading tall tales with everyone he meets. When he asks Jane why he never gets to be the bad cop during an interrogation, she retorts, "Because you can never keep a straight face." Jeff is also not above taking on what he considers "zero-risk" ventures, such as foiling an armed bank robbery or taking on muggers in a knife fight with daughter in tow. He generally doesn't use firearms, as he is a poor shot (He had to blackmail an FBI official to qualify at the firing range.) He is fluent in several languages, an able fighter, an expert interrogator, and master strategist, able to size up any situation and come up with the perfect plan.
Paulina Novacek (Fiona Shaw), former head of the Czech Secret Police, left Prague two jumps ahead of the firing squad at the end of the Cold War. Since Czechoslovakia has no extradition, Novachek can only be arrested if she goes back voluntarily. She has since become an international arms dealer who steals weapons and chemicals to sell to terrorists. Though Novachek has the appearance of a vain and silly countess who surrounds herself with henchmen, she is as dangerous and unstable as the experimental "C-22" plastic explosive she has recently stolen from the United States Army. She has encountered the Blues once before when they disrupted her operation in Budapest.
Muerte (Stanley Tucci), a cheap mugger who preys on tourists in New Orleans' French Quarter, fancies himself to be a fearsome outlaw to the point of adopting the alias of "Muerte," the Spanish word for "death." He can be often heard shouting, "My name is Muerte! My name is death!" His weapon of choice is a switchblade, which he brandishes as a sword. Having committed a number of muggings with his partner Ozzie (Dave Chappelle), he has the bad fortune to encounter Jeff Blue who is out with his daughter for an evening stroll. Though Jeff readily complies to their demands for his wallet and watch, Muerte is incensed that Jeff shows no fear of him, and continually calls him "Morty." His constant attacks usually leave him unconscious and missing a tooth, as he stalks the Blues for revenge.
Lieutenant Theodore "Ted" Sawyer (Obba Babatundé) and Detective Sergeant Halsey (Larry Miller) of the New Orleans Police first encounter the Blues after the foiled mugging, though Jeff denies any involvement. Sawyer takes an instant dislike to the Blues, particularly Jeff, who insists on calling him "Ted," instead of "Lieutenant." Halsey, on the other hand, remains congenial and good-natured to a fault. As the Blues pursue Novacek, Sawyer and Halsey attempt to keep up.
Vern and Bonnie Newman (Tom Arnold and Park Overall) are a vacationing couple with their baby boy Jonathan in the same hotel as the Blues. The Newmans find the Blues to be exciting and charming, and have no idea to their true identities, despite being told point-blank that the Blues are former spies. Vern and Bonnie bicker constantly, even taking opposite sides on naming the Blues' baby daughter.
Frank (Richard Jenkins) is the Blues' agent handler for an unnamed covert organization. Though Frank threw a baby shower for the Blues and got them a Fisher-Price Barnyard (which Jeff plays with constantly), he keeps them in mind when the C-22 is stolen, reminding them, "You said if we were ever in a critical situation... you'd help out a little." An indication of how valued the Blues are as agents is that the White House sent a crib blanket to the baby shower.
Jane Louise (or Louise Jane) Blue (Michelle Schuelke) is a bright and happy 11-month-old who seems unfazed by knife fights and gun battles. At Jeff's insistence, the Blues find that she has a taste for jambalaya. Jeff hints several times that their daughter will learn the family business, such as when they break into a dead man's house during the night: "See Mommy picking the big lock? Someday, when you're a big girl, she'll teach you how to pick locks. Of course she may still be picking this one."
Ozzie (Dave Chappelle) Muerte's accomplice as a mugger in New Orleans. Ozzie attempts to mug Jeff Blue with Muerte in the beginning of the movie but gets knocked down and pretends to be knocked out when he sees Jeff's quick dispatching of Muerte.
Undercover Blues received mixed to negative reviews from critics, as the film holds a 35% rating on Rotten Tomatoes.