The Legend of Zeta

The Legend of Zeta is an unmarked quest, and the only freeform quest, in the Fallout 3 add-on Mothership Zeta. To collect the achievement, one must collect all 25 of the locatable alien captive recordings found aboard the mothership. Upon completion of this quest, the achievement/trophy Alien Archivist will be received.

Detailed walkthrough
Without backtracking, the order is roughly:
 * 1) Holding cells (6 total).
 * 2) * The first three {14, 15, 22} are together to the east of your initial cell on the first level. Don't follow your new friend.
 * 3) * The next three {3, 1, 12} are again found together in the room just east of the quest map marker, and north by north-east of Sally's holding cell.
 * 4) Engineering core (4 total)
 * 5) * The locations of the four recordings on this level {5, 6, 7, 8} correspond to the four individual stasis controls on the pods of the samurai, astronaut, medic, and cowboy.
 * 6) Waste disposal (via maintenance level) As mentioned above you must get this before entering the airlock (1 total) (note that you can do step 3, 4, and 5 in any order).
 * 7) * The recording {19} is in the south-west portion of the waste disposal map, to the right as you enter the 'ambush' room (after Sally mentions their radios going quiet, after traversing the string of rooms she unlocks for you one-by-one). Also, you will want to pick up the samurai's sword, which is located in a closet that is full of medical supplies that you encounter just before you teleport from this area.
 * 8) Cryo lab and cryo storage. As mentioned above you must get these before entering the airlock. (6 total; 2, 4, 11 and 17 in the former, 16 and 25 in the latter).
 * 9) * Before you can get the first recording {4} inside the cryo labs, you must activate a button titled "Release Mechanism". This release mechanism is in the first room of the labs on the left side of the transparent floor. The recording is down a short flight of stairs to the right of the level entrance, inside a room blocked by a blue stasis field with other earth-based misc loot. The second recording {2} is inside a stasis room containing two aliens and a ghoul in stasis - the room south on the map, not the north one across from it. The third and fourth recordings {11, 17} are inside the room with Elliott's squad, or, if you do not have Elliott with you, three wastelanders on operating tables. The doors are locked behind you once you go through the round door with the conic hologram hovering over it. Make sure you get all four from the cryo lab before progressing through this door to the cryo storage entrance. Teleporting back from storage locks doors behind you in the lab. If this happens, in order to get the recordings you will need to teleport from the lab to storage, and backtrack to the doorway from storage to the lab.
 * 10) * The first recording inside cryo Storage {25} is just to the left of the entrance. The second {16}, is in the south-west corner of the big room, downstairs.
 * 11) Research lab (via cargo hold). As mentioned above you must get these before entering the airlock (2 total).
 * 12) * The first recording {13} inside the research lab is in the "vehicular" (containing several pre-War cars and a human spacecraft) section on a four-sided console near the human spacecraft. The second recording {21} is located in the first Giddyup Buttercup room; it is in the central portion of the room near four alien consoles, on the opposite side of the room as a closed door containing a healing archway.
 * 13) Weapons lab (1 total).
 * 14) * The only recording {24} in this location is inside the lab control room. Do not miss the fourth-largest group of 87 alien power cells on a shelf inside this room.
 * 15) Experimentation lab (3 total).
 * 16) * The first recording {20} is inside the room with the two large tentacle examination chairs, directly across from them. The second {10} is on the north wall of the observation room. The last {9} is inside the room being observed, next to the abominations on the operating tables.
 * 17) Biological research (1 total)
 * 18) * The penultimate recording {18} is in the northern-most area of the bio-lab, on the second floor, in a depression ringed with computers. Before or after you reunite with Sally and the others at the end of this level, do not miss the largest stash of alien power cells on Mothership Zeta (109), located in the adjacent room (after first disabling the door barrier)--down the stairs, near the entrance to the bridge. Make sure that you have 24 out of 25 logs before leaving this area. After you leave for the Death Ray Hub, the teleporter will break and you will not be able to access this area again.
 * 19) Living quarters (1 total)
 * 20) * The last recording {23} is inside a small room at the opposite end of the hallway from the samurai. When you enter, continue down the hallway. You will enter a large room, continue heading south until you come to a hallway, at this hallway turn right and keep going until you see a room with tables and chairs, it looks like an alien dining room/cafeteria, as soon as you enter this room it will be on your right. Alternately, it can be found in the south-western room on the map, due south-east of the exit to the bridge.

Behind the scenes
The name for this quest is a reference to the popular video game series, The Legend Of Zelda.