The Fringe Division is flummoxed by a case where victims are inexplicably disintegrated into ash. Agent Broyles takes a particular interest in the case, as it is revealed that he worked on this case in the past, and had been so involved that it cost him his marriage. The investigation reveals that Fringe science extends all the way to Russia.
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Broyles had unsuccessfully worked this case in the past and so he was already familiar with the details.
The Observer can be seen at the airport as Broyles speaks on the phone to Olivia.
The large "W+" in the Washington Hospital signage in 'Dream Logic' is shorthand for the W (weak force) boson. Weak bosons, a theoretical product of weak interaction and positron emission, foreshadows the condition of the ill cosmonaut in 'Earthling'.
The patient record for 'CARSON, D' in 'Earthling' foreshadows Dr. Carson's involvement in the cloning experiments in 'Of Human Action'.