Fringe: Season 3, Episode 11

3 - 11: Reciprocity

The Fringe team and Massive Dynamic have assembled the buried components of the doomsday device, speculated to be an artifact of the "First People", a lost civilization that supposedly existed until the "mother of all mass extinctions". The device reacts to Peter's presence, but they are unable to identify what triggered this. When several shapeshifters are assassinated, Fringe and Massive Dynamic attempt to locate a mole from the parallel universe using Fauxlivia's computer files to identify the shapeshifters. Walter happens across the same files in Peter's room, and finds that Peter has been the assassin. Though Peter claims he killed the shapeshifters as he refuses to no longer remain retroactive to the events, Walter fears that the device has "weaponized" him for some purpose.