Basically a 300 pages long happy end. If you are looking for a hopeful and lighthearted sequel go for it but be aware that it´s mediocre if not underwhelming. Definitely a cash grab and forgettable book. I might not be the best to judge since I didn´t remember a lot from the first book and am not passionate about it. 
Colleen Hoover added a few new plot lines to keep us entertained. My favorite and the only one I liked:

The dynamic between Atlas and the 12 year old Theo and Atlas bonding with his half brother Josh.

I don´t know, overall it just felt mid which is why I don´t have a strong opinion but I was entertained. 3 stars.