Once you get through the tutorial missions of Ben Ten(which ends with your first Decibel Concert) you will be given an option of 3 missions to then pursue. One in Grashugal Forest, another in Mt. Eisburg, and the last being in Strudelbek City. Whichever mission you choose will then set you on course for a chain of missions resulting in you automatically fighting a power spring come end of each mission chain. To fight the Forest Power Trip, you have to go through the Grashugal Forest mission chain. For City Power, it's the Strudelbek, and for Mountain Power its Mt. Eisberg.