Complex sentence: The first two incarnations each die during the episode in which they appear, while the third becomes the Doctor's full-time companion, travelling with him for the remainder of the series as he tries to uncover the mystery of her multiple lives.
Split into simpler sentences: The first two incarnations each die during the episode in which they appear. The third incarnation becomes the Doctor's full-time companion, travelling with him for the remainder of the series as he tries to uncover the mystery of her multiple lives.