The Disappearing Act

The Disappearing Act is a detective case file quest in Fallout 4.

Quick walkthrough

 * Accept the quest from Nick Valentine's office
 * Search Earl Sterling's house for clues
 * (Optional) Obtain the key to Earl Sterling's house (Vadim Bobrov was his friend and will give it)
 * Investigate the Mega surgery center
 * Obtain the key to the surgery cellar (obtainable from Doctor Sun in various ways (see below) after either examining the blood stain or talking to him and passing two Charisma checks: one easy and one medium)
 * Investigate the surgery cellar
 * 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 or head to the Dugout Inn and speak to Vadim Bobrov to obtain the key.
 * Next go to Earl Sterling's house look at the sofa and to the left side of the sofa is a receipt named: "Surgery Receipt"
 * 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. Then, there are a few ways of getting into the cellar: (if examined the blood stain:) pass a hard speech check (if one didn't examine the blood:) pass a choice of two medium speech check or bribe him for 100 caps (both cases:) pick the advanced lock into the cellar or pickpocket the key from Doc 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.