Springtrap, who once was Purple Guy, killed Puppet when he was a little girl (don't ask questions about my logic). But ever since they've been Animatronics, there's been something going on between them.
In the title it says Mangle, that's because it's a bisexual story. The main character is Foxy. His main goal is to find who he truly loves. But to do that, Foxy needs to break up the war between Mangle and Bon.