The Nuka-Cola Challenge

The Nuka-Cola Challenge is a side quest in Fallout 3 as well as an achievement/trophy.

Quick walkthrough

 * 1) Take Sierra's Nuka-Cola tour.
 * 2) Collect 30 bottles of Nuka-Cola Quantums.
 * 3) Hand the 30 bottles over to Sierra.

Detailed walkthrough
Sierra Petrovita lives in the isolated Girdershade and is incredibly passionate, although some may would call it insane, about everything Nuka-Cola. Upon talking to her, she offers to take you on the "Nuka-Cola tour," which consists of viewing the large amount of Nuka-Cola memrobilia she owns. If you agree to take the tour, she will show the player around her home, discuss various Nuka-Cola facts and talk about some of the items in the room. After the tour, she gives the player an ice cold Nuka-Cola.

Talk to Sierra
After completing the tour, Sierra mentions that her tour isn't actually complete because there is no Nuka-Cola Quantum. She asks the player to gather the bottles throughout the wastes and bring them back to her; she even offers 40 caps per bottle for your trouble.

After exiting her house, Ronald Laren will ask the player to instead give him the Nuka-Cola Quantums, as he hopes that giving them to Sierra will improve his chances of "plowing her bean field." He offers to pay the same price for the Quantums, though a successful Speech check will cause him to double the payment. However, if you choose him over Sierra you will lose the chance for Schematics - Nuka grenade that you would have gotten from Sierra. If you have the Black Widow perk, you also have the option to offer a threesome in exchange for him finding the bottles, causing him to eagerly run off to his death. His body can then be found inside the main entrance of the Nuka-Cola Plant, or somewhere on a more or less straight line from Girdershade to the plant if he happens to be killed on the way. If the player escorts Ronald and he successfully reaches the Nuka-Cola Plant, he will simply stand in the lobby. If the player then leaves the area and re-enters, Ronald will be dead.

Recover 30 bottles of Nuka-Cola Quantum
Nuka-Cola Quantums are particularly rare items. There are only 94 loose in the base game (plus a 10% chance of finding one in a Nuka-Cola vending machine) and only a dozen or so more with all the add-ons. They are clearly visible due to their distinctive blue-violet glow, but remain difficult to find when scrounging. For a list of bottle locations, see Nuka-Cola Quantum.

Of course, the Quantum Chemist perk makes this whole task much easier; for every ten regular (and much more common) Nuka-Colas in ones inventory, they are automatically converted into a single Nuka-Cola Quantum.

Option: Recover the shipping manifest
A good place to start looking is the Nuka-Cola plant. Aside from a few Quantums, you can find a terminal entry with details for shipments of Nuka-Cola Quantum. You can get the code for this terminal from Milo, shipping foreman by either passing a Speech check, killing him, obtaining an employee badge by finding the note Welcome to the Nuka-Cola Family! or by using a dialogue choice obtained through the Robotics Expert perk. The code is used in the computer terminal in the room where Milo is found. After using the code, select the option to retrieve the shipping manifest.

The manifest tells the player of three locations where shipments of the Nuka-Cola Quantum were in the process of delivery; these being the shopping center at Paradise Falls, Super-Duper Mart, and Old Olney grocery.

At the Super-Duper Mart, the shipment of Nuka-Cola Quantum can be found in the back storeroom near the fallen shelves next to the protectron. There are three of them. It is in the same room as the Emergency Supplies.

In Paradise Falls, five Quantums can be found in Eulogy's pad underneath the stairs next to a bed. Be careful when stealing these: if you are seen, the whole town will turn hostile toward you. You will not lose Karma for stealing them.

When investigating Old Olney, you will discover that the shipment of Quantum never made it there. The skeletal remains of the driver can be found on the steps outside a building in Old Olney along with the Nuka-Cola accident report. The note reveals that there was an accident and the driver abandoned the truck to look for help (which never came because the bombs fell). The truck with its Nuka-Cola Quantum can be found by exploring the highways surrounding the town. There are about five Quantums in the truck and about 28 regular Nuka-Colas. It is found about halfway between Old Olney and Chaste Acres Dairy Farm.

You can return bottles of Nuka-Cola Quantum to Sierra one by one or in one large delivery. Regardless of delivery method, Sierra will only reward you with the Nuka grenade schematics after receiving a grand total of 30 bottles.

Behind the scenes

 * The name of this quest is a reference to "the Pepsi Challenge," a taste-test marketing promotion run by Pepsi Co. since 1975.
 * If Sierra is killed, and Ronald is spoken to, a Speech check can be passed in which the player blames the Nuka-Cola Corporation for her death. Ronald will assume it's because the Nuka-Cola machine he got for her had a label saying; "Warning if you tamper with this unit you'll have to answer to the Nuka-Cola Corporation". This dialogue is likely a reference to a scene from the film Dr. Strangelove involving tampering with a Coca-Cola machine.
 * The login code given by Milo the foreman is a reference to the USS Reliant from Star Trek 2: The Wrath of Khan.

Bugs

 * When Milo, the shipping foreman initially approaches the character asking for confirmation instead he just opens dialogue saying "What is it now? Sir/Ma'am" (Depending on player gender). Any further attempts to open dialogue result in the same phrase repeated over and over. This may cause "The Nuka-Cola Challenge" to be marked as completed and Goalie Ledoux will not appear at the Red Racer Factory.
 * When a player enters Sierra's house she may start to flee. She may run outside and if you follow her she may return to normal and try to reenter her house or may remain in a panic, making the quest impossible to both start and complete. It may be possible to work around this bug by saving before entering Sierra's house. If you find she is fleeing, then reload the save and scout the surrounding area for any creatures that may have spawned in or near Girdershade. Kill them and then enter her house. She should no longer be fleeing. It is also possible to talk to her as normal while she is fleeing if the interaction is timed correctly.