The Nuka-Cola Challenge

The Nuka-Cola Challenge is a side quest in Fallout 3 connected to the Nuka-Cola soft drink company and its prototype beverage, Nuka-Cola Quantum. It is also an Xbox 360 and PC achievement and a PlayStation 3 trophy.

Take Sierra's Nuka-Cola tour
To start this quest, find Sierra Petrovita at Girdershade, which is located northwest of Jocko's Pop & Gas stop and southwest of Smith Casey's garage.

Upon greeting the Lone Wanderer, she offers to give the "Nuka-Cola tour," which consists of viewing the large number of Nuka-Cola collectibles she owns. If the Lone Wanderer agrees to take the tour, she will show them around her home, discuss various Nuka-Cola facts and talk about some of the items in the room. The tour can be paused after each exhibit, as Sierra won't move unless the player moves more than the camera. After the tour, she gives the Lone Wanderer an Ice Cold Nuka-Cola. Immediately after the tour, she'll talk about the Nuka-Cola Quantum. With a Speech check, one can uncover that Sierra intends to give the Lone Wanderer the schematics to the Nuka-Grenade upon completing the challenge.

Talk to Sierra
She then asks the Lone Wanderer to collect Nuka-Cola Quantums and says she will pay 40 Caps for each bottle.

Waiting outside her house, Ronald Laren will ask the Lone Wanderer 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, by choosing him over Sierra, the Lone Wanderer will lose the chance for Schematics - Nuka Grenade that would have been received from Sierra. With the Black Widow perk, a female Lone Wanderer has 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 Lone Wanderer escorts Ronald and he successfully reaches the Nuka-Cola plant, he will simply stand in the lobby. Leaving the area and re-entering will cause Ronald to die.

Recover 30 bottles of Nuka-Cola Quantum
Nuka-Cola Quantums are 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 add-ons. They are easy to see because they glow (only if HDR is enabled on the PC version). For a list of bottle locations, see Nuka-Cola Quantum.

Of course, the Quantum Chemist perk makes this whole task much easier, especially when in Sierra's house. She has a large amount of Nuka-Cola all over the place which could be stolen and automatically converted into Quantum, though this will gain negative Karma.

Recover the shipping manifest (optional)
A good place to start looking is the Nuka-Cola plant. Aside from a few Quantums, a plant terminal entry can be found with shipping details for Quantum. The code for this terminal can be obtained from Milo 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 accessible with 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 details three locations where shipments of the Nuka-Cola Quantum were in the process of delivery: 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 pharmacy supplies.

In Paradise Falls, five Quantums can be found in Eulogy's Pad underneath the stairs next to a bed. Stealing them does not cost Karma but getting caught will turn the whole town hostile.

Investigating Old Olney makes it clear 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 5 Quantums in the truck and about 28 regular Nuka-Colas. It is found about halfway between Old Olney and Chaste Acres Dairy Farm, southeast of a Red Rocket truck stop.

Just for the Taste of It (optional)
Just for the Taste of It is an optional unmarked quest available only before and during The Nuka-Cola Challenge.

Rewards

 * Sierra pays for the Nuka-Cola Quantums (40 caps each). Once given enough to fill her Nuka-Cola vending machine, she rewards the Lone Wanderer with Schematics - Nuka Grenade and +300 XP. In the future, she will also make a Mississippi Quantum pie if the right ingredients are brought to her.
 * Giving the bottles to Ronald Laren results in a loss of Karma and double the caps (80 caps each) if the Speech check succeeded, otherwise it remains 40 caps for each. Once the quest is complete (30 have been handed over), he goes to Sierra's house straight away and starts making moves; only to have Sierra completely misunderstand his advances.

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 character 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

 * Upon returning, Sierra will be gone, not found anywhere. Reloading or fast traveling seems to fix this. This may also happen after the player completed the quest.
 * 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 character enters Sierra's house, she may start to flee. She may run outside and if followed 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 one finds 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.
 * If the player character has a companion when entering Sierra's house, they may immediately turn hostile against Sierra for no apparent reason (with no hostiles outside) and hence, can cause Sierra to be killed, failing the quest. Does not always happen, though. This can be avoided by telling the companion to wait outside.
 * Sometimes, if one has enough Nuka-Cola Quantum to complete the quest "The Nuka-Cola Challenge" before talking to Sierra, and gives them to her in the same dialogue as the quest is given, then she will take them out and give hand over the caps but no schematic. The quest is then completed. On PC, this can be fixed by using console commands to add the item.