Plugged in shadow meanings

I'm sorry, what do you mean by "plugged in"?

Example of the use a shadow:

Inside, a gravel path wound through desolate grounds to a huge clump of a house, square and prosaic, all plunged in shadow save where a moonbeam struck one corner and glimmered in a garret window. The vast size of the building, with its gloom and its deathly silence, struck a chill to the heart.