The Disappearing Act

The Disappearing Act is a side quest in Fallout 4.

Detailed walkthrough
The quest becomes available after Unlikely Valentine has been completed. To begin the quest, read the Earl Sterling case notes which can be found inside the Valentine Detective Agency. The Sole Survivor should begin the investigation by searching Earl Sterling's house. There is a Novice level lock on the door which can be picked. Alternatively, the Sole Survivor can speak to Vadim Bobrov at the Dugout Inn to obtain the key to Earl's house. Speaking to Scarlett or Yefim Bobrov at the Dugout Inn can also provide speech check opportunities and additional information regarding the case. When talking with Scarlett, she will hint that Earl was eager to get aesthetic surgery.

Once inside Earl Sterling's house, search near the sofa. There is a surgery receipt on the floor to the left of the sofa. If Nick Valentine is the Sole Survivor's current companion, he will eventually find it and ask the Sole Survivor to "take a look over here" when he does. The surgery receipt is for facial reconstruction surgery at the Mega surgery center. Head there to continue the investigation.

Learn more about what happened to Earl by examining a bloodstain near the entrance to the surgery's entrance or by speaking to Doctor Sun. The Sole Survivor can ask Doctor Sun about the surgery receipt they found in Earl Sterling's house (easy Charisma check) and about Doc Crocker (medium Charisma check). All clues point to the surgery's cellar which is locked.

To access the cellar, either pick the Advanced lock, pickpocket the key from Doctor Sun or convince him to give you access to the cellar. Doctor Sun can be convinced or threatened (medium Charisma check) or bribed (100 caps) to give the Sole Survivor access.

Upon entering the cellar, the Sole Survivor will come across Doc Crocker berating a corpse on the ground - what remains of Earl Sterling. The Sole Survivor can either attempt a Charisma check to end the conflict peacefully or attempt to arrest Crocker. Both options, if successful, will lead to Crocker injecting himself with a lethal serum and committing suicide. Alternatively, the Sole Survivor can just kill Crocker. After Crocker has died (by whatever method), Doctor Sun will enter the cellar and demand an explanation for what is happening. Speak to him and give him any explanation. He will request the Sole Survivor leave so he can file a security report and "clean up" the mess that has been left behind.

Return to Ellie Perkins at the Valentine Detective Agency to complete the quest and receive the approximately 200 bottlecap reward.

Companion reactions
Note: MacCready will only like violence before a certain point in his personal quest, after which he will instead like the peaceful resolution.

Bugs

 * If you are high enough in level for the Far Harbor mission "Far From Home" to show up, you might have difficulty in getting Ellie to recognize that you have completed this mission. It appears her priority is to get you to take the new mission rather than pay off and close this one.
 * Speaking with Ellie and accepting the Far Harbor quest first, then speaking to her again will make her ask about the current investigation and pay for its completion.
 * If attempting this quest when the Mega surgery center is not open, Dr. Sun will be there but the objective to talk to him will not appear after Crocker dies. Sun will still be waiting for you once you exit the basement but attempting to initiate conversation will only cause the 'This individual is busy' message to show and he will remain immobile, potentially blocking your way from the area. If you can get past him, completing the quest by talking to Ellie will return everything to normal.