Perry the Platypus is Phineas and Ferb 's pet .
The series follows a standard plot system ; running gags occur every episode , and the b-plot almost always features Perry the Platypus working as a spy ( `` Agent P '' ) for OWCA ( the Organization Without a Cool Acronym ) , to defeat the latest scheme of Dr. Heinz Doofenshmirtz , a mad scientist driven largely by a need to assert his evilness .
Phineas and Ferb