夏幻の恋（Kagen no Koi）

That night, they were attracted to each other in that mountain.

Daichi, a university student researching Japanese entymology, sets out on a trip over summer vacation to directly observe the rare butterflies said to live in the mountains. After renting a place to live from the town hall, he heads into the mountains to study the butterflies. That day...

Amidst the pelting rain, he finds the elusive phantom butterfly he's been searching for. He couldn't let it get away, and so he pursues it. At the end of his chase, he comes upon a mansion, more dilapidated than anything he'd ever seen before, but amazing nonetheless. Seeing the butterfly enter the mansion, his chase leads him to the front, where the gate, previously shut tight, swing wide open as though to welcome Daichi.

Daichi steps in. As he explores the mansion's edge, he comes upon an ocean of flowers behind the building and a young man standing alone amongst the blossoms. In the rain, the man plucks a bouquet that fills both his arms, and disappears into the mansion. Curiosity piqued, Daichi approaches the place the man had disappeared to and peers into the house. There, he spies the man he'd seen before together with another young man.

Surprised, Daichi hastily runs away. But the image won't leave his mind.

The next day, he tries to find the place again, but he can't find it no matter how hard he tries. Out of options, he goes to leave the mountain when a thick fog envelopes its slopes. Daichi wanders through the haze when, once again, he comes upon the young man from the previous day.

Daichi approaches the man to try to ask for a way down from the mountain, but the other informs him that scaling the slopes in the fog is dangerous, so he invites him into his place.

Awaiting them there is the other man.

The two welcome Daichi, who ends up staying over for some time.

However, it never occurred to Daichi that this would result in him being shut in the mansion.

And here, during his stay, Daichi experiences pleasure and love, as though all were a summer's illusion.

[From Aarinfantasy Forums]