A swim in the rooftop "lake" leaves Taromaru covered in nasty green algae, leaving Yuki to give him a bath and the rest of the club to take care of her equally dirty laundry. As the four stand under the sun watching the clothes dry, the conversation turns to "graduation" and their plans for the future. Later that night, a bout of insomnia leads Mii-kun and Rii-san to search for the lock to Megu-nee's secret key.

What could possibly have possessed Yuki to make her charge into the club room brandishing a deck brush and shouting about cleaning up? The brilliant idea to turn the rooftop lake (a.k.a the water storage tank) into a pool for the day! The girls carefully transfer the lake's fish to buckets, scrub out the algae and pond scum, and let their troubles slip away for the day as they enjoy their new pool.

It's pouring rain. The School Living club girls are gathered in the club room, but things take a worrying turn when Taromaru doesn't show up to eat. Yuki and Mii-kun search the club room floor while Kurumi checks the other levels. The shovel-wielding senior quickly picks up Taromaru's tracks, following them down to the first floor...beyond the barricade.

THEY flood into the school as if attempting to take shelter from the torrential rain. The power has been knocked offline. The School Living club room is under siege. Kurumi is unconscious after one of THEM bit her. All alone, Rii-san prepares to keep her promise. Meanwhile, Mii-kun prepares to go to the emergency shelter and find the vaccine, but the thought of having lost Taromaru to THEM drains her will to continue. Just as one of THEM reaches out to grab her, she's saved...by Yuki?!

The School Life Club is in grave peril, as swarms of THEM flood the school.