The Combat Zone (quest)

 is an unmarked quest in Fallout 4.

Quick walkthrough

 * 1) Kill the raiders.
 * 2) Speak with Tommy Lonegan.
 * 3) Speak with Cait.

Detailed walkthrough
Upon entering the building, you will come upon the end of a cage fight in progress with Cait and an unnamed raider, with several raiders watching on as Tommy Lonegan provides commentary. As you walk further in, the raiders will instantly become hostile and attack the Sole Survivor.

After clearing the area, you will find Cait and Tommy hiding inside of the cage on stage for their protection, and after approaching Tommy he will be both grateful and enraged that his only form of clientele have been butchered before his eyes, and comments that he wonders if he should thank you or have his "little bird" disembowel you in front of him. After a brief talk, he realizes that Cait is a liability (and an addict), and insists that the Sole Survivor take her from him until he can straighten things out and eke out a better life for them both. He also awards you the purse of the last fight, and you are free to loot the rest of the building. From their, speaking to Cait will allow you to use her as a permanent companion.