Key to the Past

Key to the Past is a main quest in Fallout 76.

Quick walkthrough
Acquire the following holotape key fragments from various places across Appalachia:
 * Blackwater Bandits key fragment - Blackwater Mine, on the now feral glowing one Freddie Lang.
 * Trappers key fragment - On Walter Griswold's corpse in Huntersville.
 * Diehards key fragment - Obtained from the duplication terminal after finding the holotype on Margie McClintock's corpse in the Palace of the Winding Path.
 * Gourmands' key fragment - On Morris Stevens (progenitor wendigo) in the wendigo cave.
 * Cutthroats key fragment - Big Fred's BBQ Shack on the now Scorched David Thorpe.

Return to Rose so that she can produce a "master key" from those key fragments. Then search for information on Rosalynn Jeffries at the Charleston Capitol Building and finally go to the basement of the Pleasant Valley Ski Resort to find a raider cache storeroom with various loot.

Detailed walkthrough

 * Rose tasks the player character with locating five holotape key fragments dispersed among the five raider gangs. To find out where to locate the keys, the player will have to first travel to Blackwater Mine and access the terminal in the foyer. This will provide a series of clues on where to find the keys.
 * The first belongs to the Blackwater Bandits. The key is found on Freddie Lang, a glowing feral ghoul found deeper in the mine, in the bandits' encampment (the northern branch from the main tunnel). Kill her and collect the fragment. Beware of the mole miners that have also occupied the area as well.
 * The second belongs to the Trappers. Their encampment is found near the Devil's Backbone, and the Trappers note will point to the corpse of Walter Griswold in the center of Huntersville. Griswold's corpse can be identified by his camouflaged fatigues and is rigged to explode 30 seconds after being looted, in a similar fashion to a mini nuke detonating. However, a Perception +2 check when near the body allows the trap to be discovered beforehand and possibly disarmed before grabbing the key fragment. He'll usually be guarded by super mutants as Huntersville is one of their main encampments.
 * The third belongs to the Diehards. The player character has to travel to the Sunnytop Ski Lanes and find Margie McClintock's Holotape in the suite on the northern end of the building, by the terminal. The location is crawling with super mutants and their mutant hounds. Margie destroyed her holotape, but she instructed her lieutenant of what to do to still maintain a copy of the Diehards' key fragment. His terminal, in the basement, by the rental counter, will point to the Palace of the Winding Path. Collect a copy of the tape from the duplication terminal on the ground floor, in the northwestern office, near the back porch. To access the terminal, one needs to acquire the palace admin password. It can be found in a dresser located in the cultists' quarters in the eastern wing by the courtyard. Margie's body can be found outside on the balcony with a chem box next to her to indicate she died from overdosing on chems. The palace itself is crawling with Scorched, and feral ghouls during the public event Guided Meditation.
 * The fourth belongs to the Gourmands. Their encampment is found at Bolton Greens. The Gourmands note on top of the main building leads to the wendigo cave in the Savage Divide. The key is found on Morris Stevens, now progenitor wendigo, found deep within the cave, past several mirelurk kings, in a room full of mutilated corpses and bones. Kill him and collect the fragment.
 * The fifth belongs to David Thorpe. David, now turned by the Scorched Plague, can be found at Big Fred's BBQ Shack. After fighting off the Scorched that reside in the area, probably killing the foxes before dealing with the player characters, the key can be found on David Thorpe, easily recognized by his raider armor he's still wearing. Kill him, and collect the fragment.
 * Once all key fragments have been collected, return to Rose at the Top of the World and hand them to her so that a master key can be created. Rose has one final request: find Rosalynn Jeffries at the Charleston Capitol Building. Rosalynn's body is in the courthouse jail and can be located using the jail terminal to find a Doe, J. The holotape on the table explains Rosalynn was captured by the Responders, to be used as bait to lure David out of hiding and capture him. Of course, further research reveals that Thorpe, thinking Rosalynn had been killed by the Responders to get him to back off their efforts to rebuild after the Great War, destroyed the Summersville Dam in retaliation, unknowingly drowning Rosalynn in the process.
 * Once all other tasks have been completed, one is then directed to the cache beneath the Pleasant Valley Ski Resort. It is located past the offices and the souvenir shop, down a staircase that leads to the luggage sorting room. The keycard lies under the door mat before the entrance. It opens the way and gets the player characters through the laser grid (the laser grid will not open otherwise). Inside the cache is a plethora of items and the broken uplink necessary to advance The Missing Link and Early Warnings.

Bugs

 * When leaving the world mid quest, some or all of the key pieces may be gone from inventory upon logging back in, forcing the player to repeat the respective quest stages to acquire them again and being able to continue the quest. There is no known fix other than to finish the whole quest during one session to avoid triggering the bug.
 * The bug seems to be triggered most often as a result of the game being closed abruptly - by crashing, task killing, or using the ALT-F4 key shortcut. Avoiding those actions when possible and leaving the game world before quitting the game might help to prevent the bug from happening.
 * On the portion of the quest where the player character is asked to kill David, there will sometimes not be a tracking dot showing where Big Fred's BBQ Shack is.