Phineas and Matilda arrive at a pond to have a nice, refreshing swim. Phineas jumps straight in, but Matilda doesn't join him. She refuses to jump in with Phineas, pointing to a "No Swimming" sign, despite Phineas' constant attempts to convince him. He soon finds the sign is there for a reason.
As Phineas attempts to get Matilda into the water, he is viciously attacked by a group of piranhas. Then, he is knocked about like a beach ball by a pair of seals. With all this happening, Matilda tries to help Phineas, but her fear hinders her efforts. She tries to reach the pond with a small stick from behind a tree far away.
Phineas tries desperately to get away from the pond, but on his way out he is covered by a crocodile's jaw and brought back in. Fighting for his life in the crocodile's mouth, it's only a matter of time before his strength fails. After the fight, Phineas is left crawling out of the pond, missing his lower half.
As he crawls for safety, a huge whale emerges from the pond and lands on top of him. Slipping back into the pond, the whale reveals Phineas only as a pool of blood and random chunks of skin. Then, finally jumping into action, Matilda throws a life preserver to Phineas' remains, a little too late to save her poor friend.