Cappy in a Haystack

Cappy in a Haystack is a side quest in the Fallout 4 add-on Nuka-World.

Background
On their way through Nuka-Town, the Sole Survivor runs into Sierra Petrovita, from the Capital Wasteland. She promises she's on the trail of something big, which is later revealed to be the Hidden Cappy contest.

Detailed walkthrough
Between the Parlor and the Fizztop Grille, Sierra Petrovita is encountered talking to a raider. Intercepting the conversation reveals that she has come to find the reward for finding hidden Cappys throughout the park. Given that the reward was meeting John-Caleb Bradberton, the creator of Nuka-Cola, she figures that his office might contain the original recipe of the drink. She gives her sunglasses and a holotape to the Sole Survivor to allow them to help her search for the hidden Cappys. Once close to a quest marker, it disappears, encouraging a thorough search of the area.

The "hidden Cappy" figures are paintings on concrete surfaces in public areas of the park, and there are ten to be found. Without the Cappy glasses equipped, the player sees only a fuzzy Cappy figure with a blurred speech bubble; with the glasses equipped the code is revealed. Click on the revealed figure to register the code and update the quest. Note that the Cappy glasses do not work when worn under power armor. To see the code, either simply take off the helmet (one might have to re-equip the Cappy glasses even though the box in front of them is ticked off as if they are already equipped), or exit the power armor, click on the hidden Cappy, then re-enter the power armor.

Nuka-Town USA
There is one located southwest of the Fizztop Grille, on the large wall just left of the restrooms building.

Safari Adventure
There are two in the Safari Adventure. The first Cappy is through the hedge maze beneath the treehouse, against a wall on the eastern edge. From the entrance, the order of turns is: right, right, left, left and right. The second is just outside the primate house. Look for the gorilla statue to the right of the door and pass through a patch in the hedge to get to the statue's back. Cappy is on the back of the plinth.

Dry Rock Gulch
There are two in Dry Rock Gulch. The first is located on the back of a gravestone in the cemetery west of the Giddyup Kid Protectron. The second is in Mad Mulligan's Mine, on the wall of a little shack beside the waterfall.

Galactic Zone
There are two in the Galactic Zone. The first is at the entrance to the dead traders' camp around the back of Star Control entrance. The sign should be on the left side entrance, right next to the Employees Only sign at the Expert locked gate. The second is up the ramp to the left of the RobCo Battlezone. Turn right immediately at the top of the Spacewalk ramp, and the Cappy is on the right wall near the trash bins.

Kiddie Kingdom
There are two in the Kiddie Kingdom. One is in the Fun House in the turntable room, second door on the left when entering from the topsyturvy room or second door on the right when entering from the room with control terminal for The Fun House. The second one is located on the top floor of a destroyed wooden clocktower, just south-west of the bridge leading to King Cola's Castle.

World of Refreshment
There is one in World of Refreshment. Inside the plant, following the Nuka-Cola Quantum river. It is on the side of a two-story building across from the Nuka Wild cola sign in the wild west setting. Going in the opposite direction that was intended for the ride makes it easier to spot; it will be on the right hand just after entering the wild west setting from the "wrong" direction.

Return to Sierra
Return to Sierra Petrovita. Putting the hidden Cappy clues in the right order results in "REFRESHING," as stated by Sierra. Use the keypad to unlock Bradberton's office. In the office on the second floor, there is a button on the righthand side of the Nuka-Cola machine. This unlocks an elevator that leads down to a small vault, containing the head of the owner and inventor of Nuka-Cola: John-Caleb Bradberton.

Siding with Bradberton
He will request the machine to be shut down to end his torment of being trapped in there for centuries. If one chooses to honor his request and to shut off the power, John-Caleb dies and the locked door will open. Behind the door is a Nuka-nuke launcher, Nuka-nukes, the Nuka-Cola formula, various Nuka-Colas and the key to Bradberton's Expert-locked safe in his office. The Nuka-Cola formula must be given to Sierra to complete the quest, and she rewards 500 caps in return, although she is conflicted about the choices that have been made.

Siding with Sierra
Sierra on the other hand wants to keep him alive, offering that she can keep John-Caleb company to keep him from being lonely. She offers the Nuka-World jumpsuit as a reward.

Behind the scenes
The name of the quest is a derivation of the phrase "needle in a haystack."

Bugs

 * Hidden clues may fail to activate once the player character has found them.
 * On consoles, this can only be resolved by reverting to an earlier save.
 * On PC, one can use the console command to complete the search for the clues.
 * After taking the elevator to the vault and following Sierra up the stairs, Sierra can become stuck in the corner of the stairwell or on the walkway.
 * To fix this, leave and re-enter Bradberton's office, then return to the vault. Sierra should be found talking to Bradberton's head, and the quest can be continued.
 * PC players can also use the console command to teleport her out of the corner, and she will continue her path towards the door.
 * Once the inner door to the vault is opened, Sierra may stand in place instead of entering the vault.
 * This can be fixed by closing the door again, which should then trigger Sierra to continue normally.
 * It seems bashing her with a gun works as well. However, she might turn hostile to the player. In this case, the player should holster their weapon.
 * If the player tells Sierra that they plan to kill Bradberton, the conversation will be repeatable as long as they don't throw the switch. Companions influenced by this dialogue option may react every time this option is chosen.
 * Even if the player chooses to side with Bradberton and kill him, the final Pip-Boy log after completing the quest will say "Despite Bradberton's wish to be etherized, I accepted Sierra's offer of a reward to keep him alive."
 * Even after turning off the power, and killing Bradberton, his eyes will continue to track the player as if he were alive.