After completing The Secret of Cabot House: Special Delivery, head back to Cabot House. Jack's mother, Wilhelmina, makes an appearance and asks you to track down her daughter, Emogene. Head to The Third Rail bar in Goodneighbor, the location of your only lead.
When you arrive, speak with Whitechapel Charlie, who will then tell you to speak with Magnolia. After speaking with her, she'll bring in her friend Ham, who will tell you that Emogene has joined a cult based at Charles View Amphitheater. Follow the quest waypoint indicator to find it.
When you arrive at the theater, speak with Brother Thomas. Unless you're incredibly charismatic, convincing Brother Thomas to give up Emogene will prove difficult. You can try to pickpocket him for an office key. Failing that, you can pick the office lock. Then again, you could just kill him and his cultists and grab his key.
Use the key to free Emogene, who will agree to return to Cabot House. Return there and speak with Jack to complete the quest.