This post contains lots of spoilers!!
Sebastian is helping Mel adjust to becoming a vampire. She also has to adjust to the fact that her autism has been "cured," which she's not happy about.
McKenna decides that Base Camp isn't safe enough for her, so she decides to take off to Canada. Lily decides to go with her because she thinks she's holding Carter back because he's always worried about keeping her safe. Even more so now that they know that she has the regenerative gene, meaning she'll turn into a Tick if she's infected by one. Carter's friend from military camp, Ely, volunteers to accompany them. Carter goes off to Texas to save a farm that's been abandoned by their Dean. There he finds Joe (the father of McKenna's unborn baby).
McKenna goes into labor early. She successfully delivers the baby (named Josie), but there are complications and she dies. Ely and Lily barely escape Ticks with baby Josie. However, Ely unexpectedly betrays her - he tranqs her so he can leave Josie behind. She's able to defeat him, leaving him alone and tranqed in the middle of nowhere without a car or supplies. Before being tranqed, he alludes to the fact that he's working with Roberto.
Lily calls Carter using Ely's sat phone. She plans to turn herself over to a farm because they'll be able to care for Josie better than she can. Carter talks her out of it - he asks her to wait for him and he'll come get her and bring her to the farm he's working to save.
While waiting, Lily gets attacked by a Tick. It stabs her in the good with an arrow that's infected with its blood. Lily tries to convince Carter to kill her before she turns into a Tick. He refuses of course. He calls Sebastian and begs him to turn her. He refuses. But he does tell him that Roberto has a cure and that they are close to his ranch.
Lily and Carter show up at Roberto's Ranch and are surprised to see Mel there. Sebastian has sent her to kill Roberto. Mel and Lily are also surprised to see their father, who left when they were young. Apparently he's the abductura that's working with Roberto.
After Mel tries to kill Roberto, they realize that Sebastian has only been using them so he could get his revenge on Roberto. Sebastian was the one who created the Tick virus, not Roberto. Roberto doesn't have the cure. And Carter is an abductura. Carter lets Lily's father escape with her because he claims he can keep her alive until Carter finds the cure. Mel is able to kill Roberto and then stakes Sebastian (vamps aren't fully dead until they're beheaded though). Mel and Carter then take off in search of the cure.