The Shrinking Submarine is an invention of Phineas and Ferb. It appeared in "Journey to the Center of Candace".
At the beginning of the day, Phineas and Ferb were having trouble deciding which they wanted to build that day: a shrinking ray or a submarine. While visiting with them, Isabella's sash was eaten by Pinky. Ferb silently suggests that they combine the two Big Ideas to help by shrinking down and retrieving the sash from inside Pinky.
Traveling Inside Candace
Once shrunken, the intent was to have Pinky eat a grilled cheese sandwich with the submarine inside of it. Pinky got distracted and ran off before Isabella could feed him, and she chased him. Candace then came out because there was so much noise and saw the sandwich. Figuring that on an empty stomach she would "pig out" on her lunch date with Jeremy, she eats the sandwich, and Phineas and Ferb along with it.
- Ferb came up with the idea to retrieve Isabella's sash that Pinky ate.