Complex sentence: An alternate Captain Braxton arrives having detected their presence in the past and he returns them to their own time at the place they left it but unfortunately is unwilling to bring them to Earth in the future as that would violate the Temporal Prime Directive.
Split into simpler sentences: An alternate Captain Braxton arrives, having detected their presence in the past, and he returns them to their own time at the place they left it. Unfortunately, he is unwilling to bring them to Earth in the future, as that would violate the Temporal Prime Directive.