The Warrior's insertion point is just outside Junction City, which will be non-hostile. The objective is to collect all the parts of a robot that approached the town after being damaged and was subsequently destroyed by the townsfolk. Each part is in a different location, all but one of them is held by the townsfolk. Talking to Anna Guarana, the Mayor's assistant inside the Town Hall will let the player character know where each part is located, and what she thinks of your possible new mechanic.
John Levis, the Mayor, is located in the Town Hall. He tells you he is in possession of the robot head and will give it to you in exchange for killing all of the Reavers threatening his town. All of the reavers are in or around a building at the top right edge of the map. Proceed through the top entrance of the town and head towards the Reaver hideout. Be cautious as you approach as the Reavers, though much less resistant to damage compared to the super mutants, have much more advanced and devastating weaponry such as the Vindicator minigun, Sunbeam laser rifle, and P94 plasma rifle. A majority of them will be around the building's perimeter behind various walls, fences, and boulders. In addition to that, there are several Reavers on the balcony of the building with sniper rifles and M79 grenade launchers sniping at your squad. Use the cover to your advantage and only pop up to either shoot or run to the next cover. Once you get closer to the building you will be able to either shoot them from the balcony or get beneath it to avoid their fire. You can also use the windows to hit the Reavers waiting inside.
There is the body of a dead civilian at the foot of the staircase to the house, check it and take the house key. As you enter the building, kill any Reavers you may have missed and move to the door in the center of the room. This door can be unlocked with the house key, but it is also trapped. The only way through is to either disarm the trap or to just take the damage. In the next room is a few more Reavers and a workbench containing the robot torso. Now proceed up the staircase. On the second floor there will be two Reavers waiting to ambush you on the right side hall and a third Reaver sleeping on the floor of the bedroom (as well as the Reavers on the balcony if they are still alive). Once they are all eliminated, return to the Mayor for your reward, the robot head.
Inside The Pit, the town's bar, you can find Angry Rick. At first he will simply curse at you for asking for his bar's biggest attraction, the robot actuator, but after repeatedly talking to him, you will be able to intimidate him into handing over the part.
Chuck, near the entrance to the town, is selling a robot part, telling you it is from the destroyed robot, but this "part" is in fact a fake he made himself out of various items such as a toaster, wires, and a sex toy in order to make some quick cash.
Since Cooter, the Brotherhood's head mechanic, died after triggering a booby trap on a captured vehicle, you have been asked to replace him with the mechanic in Junction City, Casey Barrett. She can be found with her father at the South Eastern corner of the town (marked 1 on the map). Her father, Stephen Barrett was injured in the first Reaver attack and is lying on the table. Casey says she can't leave him in this condition so in order to recruit her, you must heal him. If you have a medic on your team you can use any kit on him, but if you do not, using stimpaks or healing powder on him will work. Once he is healed Casey says she will gather her things and leave to meet you at the base.
The best way to complete this mission is to take the fight right to the Reaver's hideout at the very start. If you start gathering the robot parts in the town first, that triggers the Reavers to swarm the town and attack the North and East gates, which often results in a lot of casualties for the people of Junction City.
Under the town there is an underground tunnel which has become the home of several giant rats. Inside are various items and ammunition, but nothing very noteworthy. The two entrances to the tunnel are either at the top left side of the map in a field, or in the closet on the second floor of the multi-platform building in the center of town.