When Tris sacrifices herself to Erudite, the line between simulation and reality blurs, leaving Tobias in a race to save Tris & solve the mystery of Jeanine's purple serum. An even bigger mystery waits outside the wall - the rest of Amanda Ritter's experiment. Allegiant alternative, Tobias POV. Based on the books, so feel free to PM Qs if you only know the movies.