Magica's Secret Plan is a plan that is triggered through the Slime. In Dangerous Currency Part 3 Gizmo Duck wished he knew more of what was going on and it showed him Magica's secret plan.
Step 1 Do Not Team Up with Traitorous Male (She Means Nega Duck)
Step 2 Team up with Awesome Female Criminals to form the League of Eve-il
Step 3 Discover Scrooge's Dummy Coperation (Quackwerks) in St Canard
Step 4 Build a secret base underneath St Canard
Step 5 Discover the Slime that turns people mad crazy evil
Step 6 Bring The Phantom Blot along and use his expertise
Step 7 Kidnap Scrooge's Accountant (Fenton Crackshell) from across time.
Step 8 Make St Canard yours first through the villains then the city itself.