Phin Yang Yo!

Phineas and Ferb create a Dimension Portal, which can send you anywhere you want. They choose to go to the dimension of the TV Show "Yin Yang Yo!", and end up having to help Yin and Yang defeat an army of creatures created by Eradicus.

Songs
[http://www.youtube.com/watch?v=1UzDh3t4ZX0&feature=related Yin Yang Yo! Extended Theme] (plays when Phineas and Ferb enter the dimension)

Trivia
Yin Yang Yo!: Phineas and Ferb are in this show's "dimension".

Ella Mental is helping Yin and Yang, though she actually is one of Eradicus's minions. Then again, Yin explains this by saying she (meaning Ella Mental) betrayed him.