When the passengers on a commuter train all die instantly of massive power loss, the team realizes that a time traveler is responsible... and that it may be impossible to apprehend him. Meanwhile, Walter writes a letter to Peter explaining his origins.

When a scientist falls dead after ingesting a lethal cloud of blue powder and his bones disintegrate in his body, the CDC suspects a biological attack. As the Fringe Team investigates further, a subject from Walter's past with ties to Olivia reluctantly agrees to assist with the intense case.

As devastation spreads on This Side, Walter, Olivia, and Sam Weiss struggle to find a way to stop the destruction and discover that one individual has the ability to allow Peter access to the Machine.