With the illness Trouble spreading it's a good thing that Doctor Charlotte Cross is in town. Meanwhile someone receives a premonition about their death and someone may not be what they seem. Spoilers ahead.
"Mortality" picks up a little later than the end of the last episode. In the interim Dwight has coaxed Charlotte into taking a drive with him to an isolated spot, which sounds perfectly reasonable given the state of affairs at the end of the last episode. I thought it odd that a medical doctor kept an easily accessible loaded pistol in her car. However Charlotte is a woman traveling alone in an unfamiliar area and may have had to drive through Cabot Cove.
Dwight dramatically reveals his Trouble to her and convinces her the Troubles are real. Charlotte hypothesizes that Dwight hasn't become sick despite exposure to infected people because his Trouble is always on. That would also explain Nathan's continued good health.
Charlotte says she needs to examine a sample of the cause of the Troubles to determine a course of action so Dwight decides to give her some of the Aether that's stashed in the police station.
But Mara and Duke also want the Aether to alter the illness Trouble. Dwight ends up with the Aether but Duke and Mara end up dealing with the apparent source of the Trouble. Duke is reluctant to kill to end the Trouble until Mara gets stabby to force his hand. She is still playing Duke like a violin.
Meanwhile Vince is somewhere (I'm assuming the state capital) trying to get the CDC out of Haven. Since he's no longer the leader of the Guard, he has to resort to old fashioned blackmail. The result of his efforts is to find out that the CDC denies having anyone in Haven.
"You and your twin sister"
Audrey encounters a man who sees how people will die when he touches them. After contact with Audrey he tells her that she and her twin sister will go at the same time. He doesn't tell her anything useful like when or where though. I'm going to go out on a limb and guess the twin sister is Mara.
Audrey is also having an existential crisis, possibly helped along by Mara's taunt from last week. She wonders if she is real or a shadow like the dancing bears. This deep philosophical moment is ruined when Nathan responds in true creepy obsessive style by telling her she's real because he loves her.
Speaking of Nathan's creepy obsessiveness, he outdoes himself when a Troubled person starts sucking the oxygen out of the hospital. Instead of staying with the squirrelly Pete to keep him safe, he rushes past lots of other people who need help to put an oxygen mask on Audrey. Pete's encounter with Duke and Mara might have gone differently if Nathan had actually done what Dwight told him to do.
While Pete's death ended the illness Trouble, Audrey is still sick. Does she have the Trouble illness or something else? Maybe that new doctor in town can help.
"Keep your mad doctor away from Audrey"
So who is Doctor Charlotte Cross?
The simplest answer is that she is who she says she is but came to Haven on her own, intrigued by the missing tissue sample perhaps, without CDC sanction. That would greatly simplify things after Charlotte meets her inevitable bad end.
But since this is Haven things could be a lot more complicated. Is she playing a long con to get her hands on some Aether as well as blood and samples from Audrey? Is she working alone or is there another unknown player in the game?