Complex sentence: The Doctor deduces that the Master was planning to steal this energy to gain a new cycle of regenerations; however, if the Eye is disrupted, Gallifrey will be destroyed and a hundred other worlds will be consumed in a chain reaction.
Split into simpler sentences: The Doctor deduces that the Master was planning to steal this energy to gain a new cycle of regenerations. However, if the Eye is disrupted, Gallifrey will be destroyed and a hundred other worlds will be consumed in a chain reaction.