Against his better judgement, Broadway bookie Sorrowful Jones accepts a little girl as a "marker," or I.O.U., for her father's twenty-dollar bet on the horse "Dream Prince." Big Steve Halloway, who owns "Dream Prince" and the Horse Shoe Cabaret, gets his cohorts to bet on "Dream Prince" to lose, because he knows that if he "speedballs" the horse with drugs one more time, the horse will die. Later, however, Steve plans to race the horse to win, and asks Jones to front him ten thousand dollars to bet. An article in the newspaper that day informs Jones and Steve's gang that Steve is under investigation because of his previous bet, and another article announces that the little girl's father committed suicide. Dubbing her Little Miss Marker, although her real name is Martha Jane, the gang decides to make her owner of "Dream Prince" so they can race the horse without suspicion. "Marky" is delighted to have a horse of her own, because it fits in with all the Arthurian legends her mother read to her before she died, and is disappointed when Steve takes the horse away. Two of the gang's thugs, Sore Toe and Canvas Back, are appointed as babysitters, while Jones acts as her guardian. Marky renames all of her new pals with names of knights of the Round Table. After Steve goes to Chicago to place his bets, his girl friend, Bangles Carson, falls asleep at Jones's apartment while singing Marky to sleep. The next day, Bangles orders new clothes for Marky and, secretly in love with Jones, advises him he could use a new suit as well. Surprising even himself, the crusty Jones pays for Marky's new clothes and buys himself a suit, reaffirming Bangles' faith in him. Bangles takes in Marky when the police come to search for her in Jones's apartment. Bangles' maid wires Steve that Bangles is seeing Jones, and Steve returns with vengeance on his mind. He interrupts a party at which Bangles, Jones and all their friends are dressed like knights of the Round Table. Bangles and Jones staged the party hoping to renew Marky's innocence and faith, which have been lost due to over-exposure to the gang's tough ways. When they allow Marky to ride "Dream Prince," Marky's sweetness is finally restored, but Steve causes the horse to rear, and it throws Marky to the floor. Jones rushes her to the hospital and drags the best children's surgeon away from his own wedding to operate on her. When Steve comes for revenge against Jones and Bangles, he is drafted into providing a life-saving blood transfusion for Marky. Feeling magnanimous after his donation, Steve bows out gracefully, while Jones destroys the speedball, thus ensuring "Dream Prince's" survival. He and Bangles rejoice in their love and Marky's recovery.