From DMP:
One evening, a boy named "Air" suddenly appears at Mizuki's window, forcibly transporting him to Neverland. Having grown up most of his life without his own father, Mizuki wants nothing to do with being a surrogate father to Air and the other children, but unfortunately stuck without a way out of Neverland, he has little choice but to stay. As he spends time with Air, however, Mizuki is gradually charmed by his boyish innocence.