The fugitive (Arthur Hohl) actually killed his man in self-defense, but an angry girlfriend (Mayo Methot) is determined to testify against him. Jimmy's mission is to make sure that Monty Barton can inherit his fortune and enjoy it as a free man, minus the 50% Jimmy will claim. The key is to neutralize the girlfriend. That can be done by getting her to marry Monty, since these are still the good old days when a wife is not allowed to testify against her husband in court. But Jimmy doesn't want to cut the girlfriend in on Monty's money. So before he makes his move on her he press-gangs Louie's dimwitted girlfriend (Alice White) to marry Monty before a justice of the peace. Then, keeping this secret, he convinces Monty's girl that she'll get a fortune if she marries Monty and effectively acquits him. Later, he'll be able to cut her out by saying the marriage was illegitimate. In case Louie's girl gets any ideas, Jimmy has her marry Monty under a false name, so her marriage won't be legitimate, either. It's actually a pretty brilliant scheme, and it works, but it all turns to ashes for Jimmy when Louie blabs about it to Joan, thus destroying the image of a reformed Jimmy that the title gent had tried to cultivate.
To atone, Jimmy makes a grand gesture of self-sacrifice. He goes to Wallington's office and surrenders his share of Monty's inheritance to his rival so it can go to the granddaughter. As it turns out, Jimmy has smelt a rat for some time. Wallington had been making romantic moves on Joan for a while, but now that he has a big payday he moves to ditch her and take a steamer across the Atlantic alone. Jimmy manipulates everyone so Joan will catch Wallington on the ship and after the requisite brawl -- much of it happening behind a closed bathroom door -- our hero gets the girl. The problem with all this is that Jimmy Corrigan is one of the most unlikable characters Cagney ever played. He never ceases to be a ruthless self-serving manipulator and a thug at heart, but there's little charm or even charisma to compensate for that. There's no reason to root for him except that he's Jimmy Cagney, and that might be enough if he really were the sort of cartoon character he seems to be here, but I think audiences knew better and I hope they recognized this as second-rate Cagney, as Cagney himself apparently did.
The trailer doesn't play the "gent" angle up so much and thus sells the picture more accurately. It's from TCM.com, of course.