Commissioner Gordon plans a "flower-in" at Gotham City City Park but is disappointed to learn that there are no flowers available. Meanwhile, the dastardly Louie The Lilac plans to "take over" the minds of Gotham City's flower children — and he begins by kidnapping one of their leaders, Princess Primrose (having placed her under the power of his nefarious Lilac Spray), an old school chum of Barbara Gordon, who knew her as Thelma Jones. The Princess kidnapping' from the flowerless flower-in has the flower children incensed and they immediately start demonstrating at Gotham City City Hall. Louie, au naturel, knew that Barabra would contact The Caped Crusader (which she did!), and rigs a trap for him and The Boy Wonder. While The Dynamic Duo converse with Barbara, Louie plants a card for Lila's Lilac Shop in The Batmobile. The Batman and Robin discover the card, decide it is a clue, and proceed Lila's Lilac shop, where Louie has prepared a pernicious treat for them: they are drugged with poisonous lilacs and a well-laced vase and left spreadeagled on a trellis in Louie's hothouse, where they regain consciousness and find themselves about to become lunch for deadly man-eating lilac plants! Barbara Gordon is tailed to her apartment by Louie's henchman Arbutus, who confronts her and yanks out the phone cord as she is about to contact the authorities. She then locks herself in her bedroom, changes into that Caped Crusadress, Batgirl, and chases off Arbutus.

Alfred Pennyworth arrives to inform her that The Batmobile has been located near Louie's greehouse and give her the address, and she mounts her Batgirlcycle and speeds there in a flash!Meanwhile, Louie takes Primrose, still under his spell, to the flowerless flower-in, where she sings his praises! When the spray wears off, The Princess spills the beans, and unlucky Louie escapes in his Flowermobile, with Primrose and her flower-powered followers hot in pursuit. Back at the hothouse, The Caped Crusader manages to free one of his feet to punt a nearby flowerpot through a window, breaking it (Holy Field Goal!). The cool air pouring in instantly destroys the deadly plant, and The Dynamic Duo leap free from the anthropophagous bloom in the greenhouse and battle the returning Louie and his henchmen Arbutus, Acacia, and Dogwood. Batgirl gleefully joinss in on The Batbrawl, and manages to defeat Louie with a can of Powdery Mildew spray that renders him moldy, decayed and moth-eaten. Louie, unwilling to be taken in, tries to throw himself into the man-eating lilac plant. After rounding up Arbutus, Acacia, and Dogwood, The Dynamic Duo rescue Louie from the greenhouse, where The Lilac was unaware that the plants have already been neutralized by cold air, and haven't done him a modicum of harm!

The Dynamic Duo then discover that Egghead has joined forces with Cossack Queen Zelda...