The Disappearing Act

The Disappearing Act is a side quest in Fallout 4.

Quick walkthrough

 * 1) Accept the quest from Nick Valentine's office.
 * 2) Search Earl Sterling's house for clues.
 * 3) (Optional) Obtain the key to Earl Sterling's house (Vadim Bobrov was his friend and will give it).
 * 4) Investigate the Mega surgery center.
 * 5) After examining the blood stain or talking to Doctor Sun and passing two Charisma checks, obtain the surgery cellar key (pickpocket Doctor Sun) or pick the advanced lock into the cellar behind him.
 * 6) Investigate the surgery cellar.
 * 7) Deal with Doc Crocker.
 * 8) Talk to Ellie Perkins.

Detailed walkthrough
The quest starts after Unlikely Valentine has been finished.
 * Pick the Novice level lock to Earl Sterling's house (subsequently considered trespassing) or head to the Dugout Inn and speak to Vadim Bobrov to obtain the key. (Speak with others in the Dugout, including Vadim's brother Yefim and Scarlett, for additional speech check opportunities.)
 * Next go to Earl Sterling's house look at the sofa and to the left side of the sofa is a Surgery receipt. If you have Nick Valentine with you, eventually he'll find it and ask you to "take a look over here" when he does.
 * Speak to Doctor Sun next to the Mega Surgery Center and pass two charisma checks (one easy and one medium, if having the receipt) or examine the blood stain without speaking to him. Pick the advanced lock into the cellar or pickpocket the key from Doctor Sun.
 * After going into the cellar, talk to Doc Crocker. You can either end the conflict by passing one of two charisma check options for to arrest or end peacefully (both will cause Doc Crocker to commit suicide) or kill him.
 * Talk to Doctor Sun.
 * Go back to Valentine Detective Agency and collect the approximately 200 Bottle Cap reward.

Companion reactions

 * MacCready will dislike arresting Crocker, but like killing him (this is only before a certain point in his personal quest, after which he will instead like the peaceful resolution).
 * Cait will dislike arresting Crocker, but like talking him down peacefully.
 * Curie will like talking Crocker down peacefully.
 * Piper will like talking Crocker down peacefully.
 * Deacon will like arresting Crocker, whether the persuasion is successful or not.
 * Hancock will dislike arresting Crocker and dislike attempting to talk him down peacefully, however he will like using violence.
 * Nick Valentine will like arresting Crocker, but dislike killing him.
 * Danse will like arresting Crocker, but is neutral towards a peaceful conclusion.
 * Strong will like arresting Crocker and dislike attempting to talk him down peacefully, however he will love using violence.
 * Codsworth will like talking him down peacefully.
 * X6-88 is ambivalent to the outcome of the quest and dialogue choices. [Update 3/20/2016: X6-88 likes skipping arrest and going straight to 'Violence.']
 * Preston Garvey will both like arresting Crocker, and talking him down peacefully, But will dislike using violence.