Manhattan press agent Sidney Falco (Tony Curtis) has been unable to gain mentions for his clients in J.J. Hunsecker's (Burt Lancaster) influential, nationally syndicated newspaper column of late because of Falco's failure to make good on a promise to break up the romance between Hunsecker's younger sister Susan (Susan Harrison) and musician Steve Dallas (Martin Milner), an up-and-coming jazz guitarist.
Falco is losing money and clients. Given one last chance by the bullying, intimidating Hunsecker, he schemes to plant a false rumor in a rival column that Dallas is a dope-smoking Communist, then encourages Hunsecker to rescue Dallas's reputation, certain that the headstrong boyfriend will reject Hunsecker's favor and end up looking bad to Susan.
The plan works, in a wayâDallas can't resist insulting Hunsecker's methods, and, forced to choose between them, the timid Susan breaks up with Dallas in order to protect him from her brother. Hunsecker, however, is enraged by Dallas's insults to him after a brief confrontation. He decides to ruin the boy after all (against Falco's advice) and wants to have marijuana planted on the musician, then have him arrested and roughed up by corrupt police Lt. Harry Kello (Emile Meyer).
It is such a dirty trick that even Falco wants no part of it, at least until Hunsecker promises to take a long vacation from his powerful column and turn it over to Falco in his absence. At a nightclub, Falco slips the marijuana cigarettes into a pocket of a coat belonging to Dallas, who is accosted by Kello outside the club.
Falco is summoned to Hunsecker's penthouse apartment, only to find Susan there by herself and about to attempt suicide. He grabs her just as her brother walks in, but Hunsecker, encouraged by Susan's silence, accuses Falco of trying to assault Susan and begins beating the physically weaker Falco. Falco pleads that he only came to the apartment at Hunsecker's request, prompting Hunsecker to tell Falco that he never called him. As Susan stops Hunsecker from further harming him, Falco realizes that Susan placed the call in order to bring the men to blows.
In a climactic confrontation, Falco reveals to Susan that it was her brother who ordered him to destroy Dallas's reputation and their relationship. Hunsecker makes a call to Kello to come after Falco, who tries to flee but is caught in Times Square by the brutal cop.
Back in the penthouse, Susan, her bags packed, acknowledges to her brother that she attempted suicide, considering death preferable to living with him. She walks out on him, saying that she will go to Dallas and tells Hunsecker that she pities rather than hates him. A mortified J.J. looks at his sister from his apartment, as she walks out into the daylight.

Question: Why does Susan break up with Dallas?
Answer: In order to protect Dallas from her brother.
Alternative answer: To protect him from her brother.