Jewel of the Commonwealth

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

Detailed walkthrough
It seems Diamond City is the best hope for gaining further information about Shaun. Getting there from Sanctuary Hills is a long trek southeast. Upon crossing the bridge into the remains of the city, look for signs pointing in the direction of Diamond City.

At the entrance, the Sole Survivor runs into Diamond City's reporter, named Piper. She cons the player character into helping her fool the guard via the intercom, to open the gate, thus letting the Survivor in.

At the entrance, speak with Mayor McDonough to get his thoughts on Piper. A moderate-difficulty Speech check will get him to tell the Sole Survivor who to talk to about Shaun, then head into Diamond City market.

Who can help the Sole Survivor in this bustling market? Most are friendly folks, and asking any of the following provides a clue to the Survivor's next move:
 * Danny Sullivan - the gate guard.
 * Pastor Clements - of the All Faiths Chapel.
 * Nat Wright - Piper’s kid sister, selling copies of Publick Occurrences.
 * Piper Wright - after being interviewed.
 * Moe Cronin - Swatters salesman extraordinaire.
 * John - the barber.
 * Arturo Rodriguez- the weapons merchant.

Speaking with any of those people will direct you to the Valentine Detective Agency. It seems the hard-boiled detective Nick Valentine has gone missing. Ellie Perkins hopes the Sole Survivor might lend a hand to help find him. Charming Miss Perkins will allow the player character to shake her down for a few caps upon delivery of her boss.