Big Boy is expanding his crime syndicate by taking over new territory and exterminating anyone who stands in his way. Dick Tracy and his girlfriend, Tess Trueheart, are at an opera performance when his wrist radio alerts him of the killings. He excuses himself and quickly heads for the 7th Street Garage.
When Tracy, in his yellow overcoat and matching hat, arrives at the crime scene, he surmises immediately that Big Boy is responsible. Tracy returns to the opera and Tess. After the performance, he answers questions from various news reporters. The camera shows a cartoon-influenced-view of the city and zooms in to the Club Ritz as a female singer.
Breathless Mahoney croons "Sooner or Later (I Always Get My Man)." Inside the club, Breathless continues her performance, accompanied on the piano by 88 Keys. As the club's owner, Lips Manlis, slurps down a plate of oysters, one of his hoods reports that several of his men were bumped off and the others are getting very nervous.
When Breathless joins Lips at his table after her performance, he compliments her singing, but she seems oblivious. (She's obviously his woman, but isn't thrilled with the arrangement). Soon the police arrive to arrest Lips for operating a gambling establishment; they also arrest Breathless. However, it is soon revealed that these are not real cops; they are Caprice's men.
As Tracy and Tess walk down the street, Tess tries to convince her detective boyfriend to settle down to a less dangerous job, maybe a desk job. Of course, Tracy isn't interested in playing it safe. As they enter a diner, they are almost knocked down by the Kid as he flees the scene of his theft of a watch. Tracy chases after the kid, but seemingly loses him.
The Kid takes the watch to a shantytown where he presents it to his ruthless guardian. Just after the man hits the boy, Tracy intervenes. The scene changes to Big Boy's hideout where he demands that Lips sign over the Club Ritz to him. Once Lips reluctantly signs, Caprice orders his hoods to give Lips a cement bath.
Otherwise known as a gruesome death of being completely covered in cement and dumped into the bay. Caprice takes Lips's girlfriend, Miss Mahoney, as his own. After rescuing the Kid from his abusive guardian, Tracy takes him back to the diner where the boy enjoys a wonderful meal. With the help of Tess, Tracy temporarily adopts the Kid.
During their meal, Tracy gets a message on his wrist radio to come to the Southside Warehouse. Once he arrives, Tracy finds remnants of walnuts, Big Boy's favorite nuts. At the Club Ritz, Big Boy choreographs Breathless and a chorus line of girls in a rehearsal of "More."
During the rehearsal, the film flashes back and forth from the song rehearsal to Tracy questioning Mumbles, one of Caprice's hoods. After questioning Mumbles, Tracy and the police head for the club to arrest Big Boy for Lips's murder. After the cops take Big Boy away, Tracy stays behind. He follows Breathless into her dressing room and asks her to testify against Caprice.
Breathless attempts to seduce Tracy and he comes close to giving in to her feminine charms. She says, "You don't know whether you want to hit me or kiss me. I get a lot of that." When Tracy rejects her advances, she refuses to testify against Big Boy. Since she is the only witness, there isn't enough evidence against Caprice, so he is released.
When Tracy returns to his apartment early the next morning, Tess is asleep in the living room while the Kid is sleeping in Tracy's bed. Tess seems very uncomfortable to be in a man's apartment, so she hurries back to her greenhouse. She claims she is expecting a shipment of delphiniums. Tracy and Tess get the Kid new clothes, and feed him all day.
When they return to Tess's apartment, there is an attempted hit on Tracy, but it is unsuccessful. Miss Mahoney shows up at Tracy's office. (Apparently Tracy had telephoned her.) She is weainng a sexy dress. She says, "Thanks for calling. I was wondering what a girl had to do to get arrested." , Tracy responds, "Wearing that dress is a step in the right direction."
Breathless agrees to testify against Big Boy, but only if Tracy gives in to her sexual advances. Even though he is obviously attracted to her, Tracy resists. At the Club Ritz, Big Boy brings together several local crime bosses, including Spud Spaldoni, Pruneface, Ribs Mocca, Mumbles, Itchy, and Numbers.
Tracy has secretly followed Breathless there and informs Pat and Sam. Big Boy proposes forming an organization with him as the Chairman of the Board and them as the Board of Directors. Spaldoni refuses and leaves the meeting. When he gets into his car, it explodes. Itchy and Flattop grab Tracy and bring him to a boiler room, where Big Boy is waiting.
The Kid has stowed away on the car's back bumper. Big Boy offers Tracy a bribe. When Tracy refuses to cooperate, Big Boy instructs his hoods to kill him but make it seem like an accident. They cause the boiler to overheat. Just before it explodes, the Kid climbs through a small window and cuts Tracy loose.
The Police Chief presents the Kid with an Honorary Detective Certificate with a badge for action in the face of grave danger. However, it is only a temporary certificate because the Kid needs to choose a real name. During the ceremony, the Kid presents Tracy with a policeman's badge he had found in the trash; it belonged to D.A. Fletcher.
In a graveyard, Big Boy tells the D.A. in no uncertain terms that he owns him. Later, the D.A. tells Tracy that he has fourteen witnesses that will testify that Big Boy was at dance lessons at the time of Lips Manlis's killing. Breathless shows up at Tracy's apartment. Dressed in another slinky outfit, she asks, "Aren't you going to frisk me?"
She's worried that if she testifies Big Boy will kill her. Just as she plants a kiss on Tracy's lips, Tess and the Kid arrive from a trip to the grocery store. When Miss Mahoney leaves, Tess cries in the kitchen. A person with no face, The Blank, offers 88 Keys $5,000 to give a letter to Big Boy that guarantees him protection for ten percent of his business.
88 Keys unsuccessfully tries to convince Big Boy that he has nothing to lose and should accept the offer. While Breathless sings a reprise of "Sooner or Later (I Always Get My Man)," Tracy leads a seemingly unsuccessful raid on Club Ritz, but it is only a cover. During the raid, one of Tracy's men plants a listening device in a back room of the club.
Now Bug Bailey can listen in on Big Boy's plans. As Breathless's song continues, a montage shows the police foiling numerous crimes due to information gathered from the bug. When Big Boy discovers the listening device, he uses it to lure Tracy. Tracy meets Tess at the diner where she tells him she is leaving town.
During their meeting, Tracy gets a call that something big is going down at the Southside Warehouse. Of course, Tracy leaves Tess and rushes to the warehouse. Caprice has captured Bug Bailey and is bathing him in cement just like he did Lips. Tracy manages to save Bug. Just as Pruneface is about to kill Tracy, The Blank steps out of the shadows and shoots Pruneface.
Big Boy now agrees to the proposal that 88 Keys had offered, but only if it includes killing Tracy. Heartbroken, Tess goes to her mother's house. While there, she has a change of heart and plans to return to Tracy, but before she can tell him, The Blank kidnaps her. Alone at the club, 88 Keys and Breathless sing "What Can You Lose?," a rueful song about unrequited love.
88 Keys is obviously in love with Breathless, but she has fallen for Tracy. Tracy and Breathless meet again at the docks and once again he asks her to testify. She demands that he admit he wants her. He finally admits that he thinks about her, but he won't say he wants her because it would hurt the one he loves.
After receiving a phone call from The Blank, 88 Keys has a guy forge a letter from Tracy to the corrupt D.A. to meet him at a hotel with $10,000 because he has the goods on him. Then Tracy receives a message to come to Tess' greenhouse, but when he arrives, The Blank drugs him with hidden gas lines and he collapses.
Then The Blank, in Tracy's yellow coat and matching hat, runs into the hotel and up the stairs to a room. He shoots the D.A. with a silenced gun, then 88 keys moves Tracy's drugged body into the room to frame him for the D.A.'s murder. 88 Keys screams threats that are overheard by the hotel manager, who telephones the police.
Hotel residents and the manager then hear gunshots. Before The Blank exits by the window, he wakes Tracy with some ammonia. Tracy is arrested for blackmail and murder. Meanwhile, a lady from the orphanage comes to take the Kid. During a few scenes of him at the orphanage, he is miserable. Big Boy is back in business and thrilled.
Against a montage of various gambling scenes and Big Boy raking in the dough, we hear "Back in Business" (performed by a girls' trio: Janis Siegel, Cheryl Bentyne and Lorraine Feather). From the orphanage, the Kid calls the Chief and tells him he knows Tracy didn't kill the D.A. The Chief picks the Kid up from the orphanage on New Year's Eve to visit Tracy in jail.
The Kid tells Tracy he now has a permanent certificate because he has chosen a name: Dick Tracy, Jr. Sprung from jail by his colleagues on New Years Eve, Tracy sets out to rescue his girlfriend. The Blank calls the police and frames Big Boy for kidnapping Tess. Tracy gets Mumbles to tell him who set him up for the D.A.'s murder.
Mumbles clearly states without mumbling that Big Boy paid 88 Keys to get Tracy out of the way. At a shootout outside the Club Ritz, the police and Tracy gun down most of Big Boy's men. Big Boy finds Tess in a vault. He takes her hostage and ties her to a drawbridge mechanism when he finds the drawbridge is up and can't escape.
With some help from Breathless, Tracy trails Big Boy and Tess and confronts Caprice in the gear room of the bridge. After they fight, as Tracy is untying Tess, The Blank fires a warning shot at Big Boy. The Blank tells Tracy to pick up a pipe and kill Big Boy, but he doesn't. The Blank prepares to shoot Tracy, but the Kid tackles him. Caprice shoots The Blank.
Tracy then socks Big Boy, which sends him falling to his death in the bridge's gears. Tracy rescues Tess. Beneath The Blank's mask, Tracy is shocked to find Breathless Mahoney. She tells him that he was her only mistake, and wonders if anything could have happened between them. She kisses him just before she dies.
When Tracy finds Tess waiting outside, they embrace. Later at the diner, Tracy, Tess and the Kid share a meal. Tracy gives the Kid a wrist radio. Then he stumbles through a marriage proposal to Tess, but Pat interrupts on his wrist radio about a robbery in progress. As Tracy and the Kid leave to investigate, Tracy tosses Tess a ring and says, "You're one in a million."