It's been a while since I read the first installment and I have to admit, I found it a little tough getting back into the story because I couldn't remember what had happened in Siren. I had to simply persevere and continued with the story and bit by bit, the pieces came back. Still, I recommended a refresher with Siren before starting on Undercurrent as there are a lot of references to the former. 
 In Undercurrent, Vanessa Sands is still coping with the death of her sister who was murdered by sirens. She herself was responsible for freezing Winter Harbor, freezing and trapping hundreds of sirens in a icy grave. She's trying to get back to a normal life but now, she finds herself torn between two boys. Personal problems aside, the Winter Harbor sirens are back for revenge. Now, Vanessa must face her past and accept that she is just like her enemies -- every bit as alluring, every bit as dangerous. 
 Confusion aside, Undercurrent was a good sequel. There was advancement in the plot, action, a good dose of suspense and mystery and it all came together with good writing and pacing. Vanessa was a likeable character as she continues to grieve for her sister as well as figure out her heritage. She was portrayed accurately and written well. Other characters were given their day in the spotlight as well, especially Paige. I was glad she wasn't forgotten and she got an expanded role, so to speak. 
 The romance was well-written too. I really liked Simon and I couldn't help but feel short-charged on his behalf on the way Vanessa was acting towards him. The ending was what I expected given what had happened during the course of the novel and I really liked how it ended. I'm looking forward to reading the conclusion Dark Water. XD