Phineas and Ferb are in their backyard wondering what to do today, Isabella comes over to invite them to a sleep over at her house. Phineas accepts, but Ferb reminds him that they lost there sleeping bags months ago. When Phineas says they'd need a dog's nose to find them, he gets an idea. Phineas, Ferb, and Isabella make a potion infused with DNA from Pinky. When Phineas drinks it he gains a dog's sense of smell. They begin searching for the sleeping bags, when suddenly Phineas becomes attracted to the kitchen fridge, which Ferb and Isabella find strange. Candace comes by and gives them their sleeping bags.
At Isabella's house more strange things begin to occur with Phineas, such as he becomes really itchy and craves meat. When the kids sleep Phineas sees the full moon, leaps out of the window transforms into a werewolf, and howls. The howl wakes up Ferb and Isabella, and also Candace. Ferb and Isabella see Phineas's sleeping bag is empty, and after finding wolf hair next to the open window, Isabella deduces that the potion they made has transformed him into a werewolf. Ferb hastily makes an antidote for the potion, and they go out to search for Phineas.
Meanwhile Candace is outraged at Phineas's howl waking her up, and goes downstairs to investigate. She sees Phineas raiding the fridge, not recongnizing him, and goes to wake up her mom. When they get to the kitchen Phineas has left. Candace calls Stacy and Jenny so they can form an angry mob, exhausted from lack of sleep they reluctantly accept.
Ferb and Isabella find Phineas and chase him on the roofs, followed by Candace, Stacy, and Jenny (Werewolf Hunting). Ferb and Isabella trap Phineas with a bowl of steaks and bring him back to Isabella's house. Candace follows them, leaving Stacy and Jenny asleep.
At Isabella's house Ferb and Isabella tie up Phineas and feed him the antidote, changing him back. Candace barges into the room, and finds the werewolf is "gone". She throws a tantrum, and collapses to sleep. Ferb says, "Well this was a howling adventure."