Inside, a serial killer dubbed the Fens Phantom has built a menagerie of horrors to portray his kills to an unnamed detective on his trail. Following the holotapes left by the murderer, the Sole Survivor can learn about the killer's interest for the detective, leading to his ultimate demise.
Access to the sewers is gained via a manhole to the north of Diamond City. Take the exit to the west of the main gate and head north, turning west at the scaffold ramp into a side alley. The entrance is surrounded on 3 sides by railings, within their confines is the manhole cover, a toolbox and a skeleton.
Upon entering, turn the right at the end of the short corridor, where one can see a skeleton trussed from the ceiling with an "x" etched on the wall. Next to this a side cabinet with a toy car, toy rocketship and the first dear detective holotape sat on the top. There is also 2 wooden blocks and a toy truck on the lower shelves. Looking to the right while approaching the skeleton, a skull and arm can be seen protruding from the wall.
Turning right from the cabinet and heading down the slope, the area opens up into the sewer itself. On the left a headless skeleton is sat on the wall arch, propped up by a toolbox, holding a fishing rod and a skull fixed to the hook. Below the pelvis "xx" has been etched onto the wall. Head down into the water and turn right, following the flow of the water, which contains three feral ghouls. Halfway along the sewer path here, one can see a skeleton on the floor between a toolbox and a radio. Just above them there is a single "x" on the wall.
Exit the water from the ramp at the end and follow the path through the narrow pathway, veer to the left and a skull can be seen on a barbecue, next to the second dear detective holotape. Looking to the west of this display there is a cooler and a makeshift battery. Turn back to the east and head left upon reaching the far wall. Follow the pathway into another sewer section, making a right turn in order to head down the stairwell and into the water, which contains several barrels and two bloatflies. Take the steps up to the far left and head through the corridor on the left hand side.
Shortly upon entering the corridor, the path forks in two directions. The left hand fork leads to a dead end, blocked by a grate. However there is a matress at the top and several items of food in a shopping cart. Heading straight on, then turning left leads out into the next section of the sewers. Upon entering this section, one will see a cymbal monkey and a skeleton with a brahmin skull wearing a bowler hat sat on the ledge, glancing to ones left there is five baby strollers.
Head over the bridge and up the slope, turning right and following the wall into a new corridor. At the end of this corridor is a gas filled room with 2 mannequins and a skeleton on a pallet at the far end with a chair and side table before the,. On the table is the next dear detective holotape and a 10mm pistol. Make a turn u-turn and continue through corridor, which will open up into the upper section of the room with the strollers. Head up the ramp and follow the pathway to the right. Partway up the wooden steps and dog kennel with a human skeleton can found inside. Continue past this and turn left at the top, where the final corridor can be seen at the far end.
upon exiting the corridor, one is in a maintenance area containing several feral ghouls and a glowing one. There are several side rooms in here, the first room, which is due to south from where one enters the area, contains the final holotape, a copy of Tumblers Today and an Expert locked safe. To the far west of the main room is a red steamer trunk. heading up the stairs and following the winding linear path will lead back into the room with the skeleton fishing with a skull.
- Four audiologs, the dear detective tapes, can be found along the path with many dead bodies in peculiar postures. Each has a white 'X' chalk mark on the wall indicating the body count.
- A flamer - In a hidden alcove behind the glowing feral ghoul, behind a chain fence.
- Tumblers Today issue #2 - At the end of the zone near a safe and the last tape in the "cell" area.
- A Stealth Boy - Right next to the Tumblers Today magazine.
- An X-cell - In the first room near the fishing skeleton, across the water in a blue basket tucked behind a gray pipe and a blue pipe.
- A Vault-Tec lunchbox is at the opposite end of the tunnel from the skeleton with the fishing rod. It is in the water just past the metal ramp in a cluster of three blue crates. It may be underwater. A second one is in one of several baby strollers near the "XXXX" on the wall.
- A dog collar located around the neck of a skeleton inside a dog house towards the end of the sewer.
- In the room with radioactive barrels in the water, jump onto the horizontal blue pipe on top of the stairs to gain access to a weapon stash near an activated radio to the left. The radio is turned on and can be heard.
- If Strong is brought here, he will speak of a time he and his brother ate a human in the sewers.
- In the last room with the skill magazine, the Sole Survivor can jump off the pathway to a small alcove with a mattress, machete and a skeleton couple. This mattress cannot be slept on. Assuming the story told in the holotapes is true (especially the fourth part), this may show the dramatic result of the detective's investigation.
The Fens Street sewer only appears in Fallout 4.