Episode 16: ... I Lose Myself -

The primary antagonist, Martha (also known as the "Filthy Woman"), poisons the group's water supply with antifreeze.

It was the first season finale of the series that did not feature original lead characters Madison, Travis, or Nick Clark. Episode 16: ... I Lose Myself

For a deeper look into the events and character arcs of this season finale, you can watch this review: The primary antagonist, Martha (also known as the