The first step in this quest is to recruit Cait, one of the 13 Fallout 4 companions. You can find her in the theater district of Greater Boston, in a locale named Combat Zone. You'll have to deal with a few Raiders once you enter. After that, speak to Tommy, which will trigger the option to recruit Cait as a partner.
As with every partner, there is the potential to bond with Cait, depending on your actions in the field and during plot developments. Consult our Companions Guide for specifics on how to impress her. When your bond has strengthened, she will confide in you about her Psycho addiction and she wants you to help her get clean.
This conversation will then lead you to Vault 95. You'll find this as you head toward the southeast corner of the map (if you've reached the high radiation zone, you've gone too far). Look out for Gunners and turrets once you're inside the vault. Make your way through until you've reached the Clean Room.
Converse with Cait to begin her detoxification. Completing this process will bring you one step closer to maximum affinity with Cait and her companion perk.