The third season begins with Phineas and Ferb building a gigantic biome during a bit of bad weather to help Isabella earn a Fireside Girl patch. / Candace realizes that she needs to spend more time with Stacy, so she vows to spend the whole day with her only. But while visiting Jeremy and his mom, Mrs. Johnson, Dr. Doofenshmirtz's "combine-inator" zaps her and Jeremy, literally joining them at the hip. Meanwhile, Phineas and Ferb build a giant robot dog that causes a lot of mischief.