Complex sentence: Gerry Anderson and Fred Freiberger rallied and pitched the idea of a new series with the addition of an alien character to Moonbase Alpha, who would shake up the dynamic of interaction on the Moonbase and regain viewer interest in the United States.
Split into simpler sentences: Gerry Anderson and Fred Freiberger rallied and pitched the idea of a new series with the addition of the alien. The argument was that an alien character on Moonbase Alpha would shake up the dynamic of interaction on the Moonbase and regain viewer interest in the United States.