"Open Doors" tells the story about the love and dedication two people have with one another despite the trials and tribulations they may experience.

In the beginning a young boy (Mushishi) is being treated for a sudden illness by a grumpy, old doctor (Tsutomu Mukae - Kids on the Slope/ Maiden Rose) who is known for being a grouch throughout the village. When the boy discovers he is ill, sorrow sweeps over his household. However, his happiness is returned to him when his "Senpai" (先輩) and guardian (Katou Youji - Haru wo Daite Ita / Kusaka Touma - Fuyu no Semi) drops by for a visit.

In the present, the boy is all grown up (Iwaki Kyosuke - Haru wo Daite Ita/ Akazuki Keiichirou) and flips through an old picture book with his Senpai when the two of them visited his mother's shrine. All was well then, but soon our protagonist is diagnosed with an illness (cancer) and has to undergo a countless amount of treatments in order to prolong his life.

However dreadful, the Senpai (先輩) never dares to abandon his kōhai (後輩). Throughout the story the viewer will notice the change of heart each of the characters have, leading to the end credits when it is up to him or her to decide whether our protagonist survives this illness or not.