During the quest Kill Joy, you'll have an objective "Go to the Old Bailey District". This is just after visiting Dr V. at Haworth Labs. On your way there you will crouch through a hole in a boarded up archway where you can see police on the other side. You'll need to head left then through another hole in a metal fence.
Then you'll see a yellow cable coming up out of a small canal of water. If you follow the cable and switch it on, you will electrocute the plague NPCs in the canal. Once they are dead, switch off the switch and jump down in the canal, follow it to the end where you will find 2 contraptions next to one another. Pick them up to unlock the achievement.