It's just another awesome day of summer vacation for Phineas and Ferb, and this time they're ready for an aquatic adventure! The stepbrothers decide to build a giant mechanical squid and travel to the depths of Danville's deepest lake to look for buried treasure. Meanwhile, Dr. Doofenshmirtz has built his own underwater creature to try to defeat Agent P—and the tri-state area —once and for all! But when Phineas and Ferb's squid runs into Dr. Doofenshmirtz's invention, will it be a friendly encounter? Or will this day turn into an underwater disaster?