Unlikely Valentine

Unlikely Valentine is a quest and achievement/trophy in Fallout 4.

Quick walkthrough
You are supposed to rescue the detective Nick Valentine who went missing at Park Street station.

Detailed walkthrough
After speaking to Ellie Perkins head out of the main gates in Diamond City. Follow the Pip Boy to take you to Park Street Station. Fight your way through the gang members until you reach Vault 114. Upon entering the Vault you will come across 3-4 Triggermen armed with sub machine guns. Kill them and move on. Carefully fight your way through the vault until you come to a large open Atrium. Head to the left and climb the stairs. In front of you will see Dino. Dispatch of him and use the control panel behind you to open the door to the Overseers office. Speak with Nick, be sure to grab the bobblehead behind him on the table. After talking with Nick he will lead you out of the vault. After a few fights with Triggermen, you will be stopped by Skinny Malone and Darla. Depending on your Charisma level and how you deal with Malone and Darla, it is possible to turn them against each other or leave the vault without fighting at all.

After exiting the Vault, follow and aid Nick into getting back to the Valentines Detective Agency. You will be engaged by Raiders and mongrel dogs while returning to Diamond City.

When you arrive speak to Ellie again to claim your rewards.