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.

A series of Fringe events leads the team to the home of a woman grieving over the death of her husband, while Peter and Olivia make an effort to repair the emotional rift between them.

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.