In total this questline consists of four quests: 'Free Larry', 'The Hero's Journey', 'Close Encounters', and 'Out Of This World'. Once Larry is free, he will give you some follow up quests. You can disable it by turning off the three nearby generators (marked on the map once you discover Larry). When you find Larry, he will be trapped behind an electric barrier. He is at the 'Parker Laboratories' fast travel point, just west of Fall's End (main town in John's region).
It is given to you by an NPC called Larry, in John's region.
To get the special Magnapulse alien weapon, you must complete an optional Aliens questline.
