"Swallow Blue" is Einar Skinnarland, a native of Rjukan, the town nearest the
plant. From his hut on the Hardanger Plateau above Rjukan, Skinnarland, along
with other members of the Resistance both outside and within the plant, served
as the Special Operations Executive's eyes on Vemork. "IMI" is the codename for
heavy water. By "reprisals" after an attack, Skinnarland means the Germans
would naturally suspect and take revenge on local Norwegians, a great concern
to the Resistance fighters, who grew up in the area.