Fallout 4/Jewel of the Commonwealth

Jewel of the Commonwealth is a main story quest in Fallout 4.

Walkthrough
This quest can either be given at the end of When Freedom Calls, or when the Sole Survivor happens upon Diamond City on their travels. Getting from Sanctuary to Diamond City is a long walk south-west from Sanctuary. The wondering trader, Trashcan Carla will give directions too.

At the entrance of Diamond City, the Sole Survivor will witness a confrontation between Piper Wright, the owner of the city's local newspaper Publick Occurrences, and McDonough, the mayor of the city. Piper will use the Sole Survivor to trick the guard into open the gate, thus letting them both in.

Speaking further with McDonough, a Speech check will get him to tell the Sole Survivor who to talk to about Shaun, then head into Diamond City market. Failing this, asking most NPCs will direct the player character to the Valentine Detective Agency. However, upon arrival at Valentine's place of business, his secretary, Ellie Perkins will inform the Sole Survivor that Valentine disappeared while working on a case. Ellie hopes the Sole Survivor might lend a hand to help find him. Passing another medium speech-check, Miss Perkins will allow the player character to shake her down for a few caps upon delivery of her boss. In any case, agreeing to help will finish the quest and start Unlikely Valentine.