In Sheep's Clothing

In Sheep's Clothing is a quest in Fallout 4.

Quick walkthrough
This quest begins when the Sole Survivor travels to Diamond City after he/she has completed Political Leanings and has been banished from the Institute, or if this condition is not met, after completing the main quest line. Gatekeeper Danny Sullivan will be seen dying in front of the walkway leading to Mayor McDonough's office. He will explain that he was shot twice in the stomach by McDonough before falling off the elevator when he saw the mayor conversing with a synth from the Institute. Upon reaching the office's front doors, Piper and a security guard will be seen attempting to gain entry to apprehend him.

Detailed walkthrough

 * 1) Chase McDonough to his office. (Optional) Talk to a doctor and bring them to tend to Danny's wounds. You can also choose to use a Stimpak to heal him. If Curie is with you, she can be asked to take care of him, in which she will be happy to do so.
 * 2) Break into McDonough's office and confront him with Piper. You can either choose to lockpick the door, which is a novice lock, or use a button located underneath the secretary's desk. Piper will dislike you picking the lock.
 * 3) Decide how to deal with McDonough. Passing a medium difficulty speech check will convince him to release his hostage. He will then request that the Sole Survivor allow him to safely leave the city, giving them the option to comply, kill him on the spot, or make him stand trial, starting a fight. Piper prefers you attempt to persuade him to stand trial and free the hostage. :)

Bugs

 * If McDonough shoots Geneva during the fight, she will become hostile to everyone. This causes Piper or any other NPC to kill her, worsening the player character's relationship with Piper and occasionally turning her hostile towards them.
 * A bug may occur where Piper and McDonough start shooting at each other after you open the door. The mayor cannot be talked to in this case. A workaround on the PC version is possible if you open up the console and temporarily disable Piper. The mayor can then either be reasoned with or killed, and Piper can be re-enabled after the Mayor is gone or dead. Simply disabling combat or general AI will not work.
 * If the player character refuses to allow McDonough to exit the city, he may instigate combat with Piper, but not the player character. If this happens and the player character engages/kills the Mayor, the other NPCs (including Piper and Geneva) will become hostile.
 * There is a chance for the dialogue to randomly bug and have Mayor McDonough and Geneva speak normally, despite the hostage situation.