Talk about it here.
Nils was in charge of communications, both with crewmembers on the surface of the Moon, and with Space Control on Earth. When the Moonbase communicated with Space Control, he suspected someone was monitoring the conversation.
He was concerned about how the Gravitron's malfunctions would affect Earth, as he had a wife and family there.
When he and Benoit were checking the controls of the Gravitron, the Second Doctor, collecting samples in his search for the cause of the infection on the Moonbase, managed to untie and remove Nils' boot without him being aware of it. (TV: The Moonbase)