In a dimension separate from the human world, there exists a realm called Arche. Arche is a sacred land where all chemical elements live as humanoids with animal-like traits, each possessing their chemical properties as abilities and instincts.

Every element in Arche was born from an absolute creator god known as the Prime Element. The Prime Element is the source of all elements and the overseer of order. Though he knows the fate of the elements, he cannot force a change in their hearts.

Arche is home to the sacred Forest of the Periodic Table. Here, each element is granted a Tree of Destiny and a Flower of Destiny at birth. These are not mere plants. They are beings that reflect the element's life force, emotions, and mental state, changing from moment to moment based on the element's condition.

The forest is always filled with clear sunlight and a subtle floral scent. It possesses a mystical landscape where giant trees, glowing flowers, clear streams, and various living creatures harmonize. At night, the forest becomes even more beautiful, like starlight.

At the deepest part of the forest lies the Spring of Life, the source of all existence. The Spring of Life is maintained by the harmony and balance of the elements, regaining its strongest vitality when Oxygen and Hydrogen harmonize to create water.

A legend has been passed down in the Forest of the Periodic Table since ancient times.

On the day two elements bound by fate truly accept one another, their Trees of Destiny shall touch branches, their Flowers of Destiny shall bloom simultaneously, and a beautiful rain shall fall upon the forest.

Humans are unaware of Arche's existence. They simply worship the elements as mythical beings like gods of rain, spirits of fire, or fairies of wind.

Currently, the human world is suffering a severe catastrophe due to a multi-year drought.

Rivers and lakes have dried up, crops have withered leading to famine, and countless lives are being lost to a lack of drinking water. Forests are engulfed in massive wildfires, ecosystems have collapsed, and war and looting for water are constant.

Humans pray to unknown gods with their last hope, continuing rain rituals and various ceremonies.

The Prime Element watches all of this with sorrow. The only way to save the human world is for Oxygen (O) and Hydrogen (H) to become one and give birth to Water (H₂O). However, the two elements struggle to grow close.

Oxygen has long liked Hydrogen sincerely and always approaches first, but Hydrogen harbors a deep inferiority complex toward Oxygen, perceiving his kindness as pity.

Though the Prime Element tries to reconcile the two several times, Hydrogen ultimately pushes Oxygen away and refuses to open his heart.

The key to saving the world lies solely in the harmony of the two elements.