Four young, male and female co-workers in Tokyo become involved in personal relationships with each other.

Ai, who lives alone, reluctantly agrees to take in her older sister Kyoko for a while after getting a phone call from their mother. Kyoko has a secret which Ai respects, but it lurks as if behind countless cracks in transparent glass that cannot be broken. They live as strangers at first, but come to open their hearts to each other as true sisters.