Flower seller Uniqua is pricked by the thorn of an unusual flower while visiting Pablo's greenhouse and transforms into a superheroine. She uses these powers to defeat the Gloom Meister, an evil villain wanting to cover her town in gloom.
Uniqua is in the backyard holding a basket of flowers. She introduces herself to the viewer as Flower Seller Uniqua, a florist. She sings Everything is Rosy as she explains her job. The backyard transforms into a large city known as Garden City. Uniqua happily gives Window Washer Tyrone and Trolley Driver Tasha some flowers.
She walks to a greenhouse and greets Gardener Pablo. They get to work, but Uniqua notices a strange-colored plant and moves it to the windowsill. She screams "Ouch!" when she accidentally pricks her finger on a thorn. It starts bleeding. Pablo tells her not to worry and that it was just a thorn. Pablo walks toward the door, but Uniqua shoots flowers out of her hand. Then fireworks start shooting out as well.
Pablo sneezes when pollen comes out, and tells her that she has the powers of a flower now that she touched the unusual flower. He says that flowers need dirt and drops some on her head. He says that flowers need water and sprays some from a spray bottle on her. Uniqua begins to float. Pablo opens a window since flowers need sunshine, and Uniqua floats to the ceiling and hurts her head.
She flies around uncontrollably and falls on the floor. Her eyes spin around until she settles. Pablo and Uniqua sing Superhero Practice as Uniqua practices her flower powers. She causes a rainbow to appear and dresses up as a superhero. She flies out to help the city. Meanwhile, Trolley Driver Tasha and Window Washer Tyrone see Flower Girl flying towards them. They ask if it is a bird or a plane, but Uniqua answers no and adds "Flower Girl!". She begins decorating the town in flowers.
The citizens are amazed. Meanwhile, in a corner of the city, Austin, who is pretending to be the evil Gloom Meister, is plotting his plan to destroy Flower Girl. He shoots a ray of shadows at Trolley Driver Tasha, causing her to drive down the street and almost into the ocean because she cannot see. Flower Girl shoots flowers into the ocean and Tasha lands softly on them. The Gloom Meister shoots some shadows on Window Washer Tyrone. Tyrone cannot see where he is going and falls.
Uniqua causes a giant flower to catch him. It acts like a trampoline and Tyrone bounces several times. Uniqua tries to find the evildoer who caused the residents harm, and sees Gloom Meister return to his underground lair in the sewer. Uniqua flies in after him. She tries to find Austin (Gloom Meister) but gets herself trapped in a booby trap cage. Flower Girl tries to use her powers to get out, but they do not work. Austin comes out of a hiding spot and tells Uniqua his name, Gloom Meister.
Austin laughs maniacally and Uniqua asks if it is his true laugh. Austin answers no and adds that he only does it to look scary, and shows her his real laugh: a combination of giggles and snorts. Uniqua agrees that the first laugh was better. Austin sings Me And My Shadows. He presents Uniqua with his Gloomsday Machine and laughs his actual laugh again. He "corrects" himself by laughing the other way. He uses the machine to shoot shadows all over Garden City. Austin takes his Gloomsday Machine outside to cover more of the city and leaves Uniqua in the cell.
While Uniqua is in the cage, she sees some light coming through a window on the floor. Since her powers work with sunlight, she reaches it and uses them to get out of the cell. She grows a giant sunflower and climbs it out of the sewer. All three citizens reach the last bit of light and scream for Flower Girl to help them. Uniqua comes and destroys all the shadows. Instead of being sad, Austin begins singing This Is My Song as they argue and start battling with their powers. The citizens want Uniqua to win the battle.
Austin shoots a shadow from his Gloomsday Machine on to Uniqua and causes her to gasp and whine. Uniqua does not continue. Instead, she asks Austin to say "sneeze". She shoots pollen at him and he sneezes until he accidentally turns off his machine by bumping into it. Uniqua destroys the device by covering it with flowers.
The citizens ask Gloom Meister why he did not like flowers, and he sadly answers that he is scared of them after a lot of negotiating. They tell him not to be afraid and show him how good they smell. He changes his feelings and now loves flowers. Then Austin redubs his character, Gloom Meister, into the Bloom Meister. Austin asks to be Uniqua's sidekick, and Flower Girl accepts and invites them all over to her house for sunflower seeds.
The city transforms back into the backyard and they sing the end song. All five children open the previously closed door and shouts "Flower power!". The end credits start.