From Wikipedia :
Futakoi Alternative tells the story of Rentarō Futaba and his detective agency. One day a set of twins, Sara and Sōju, show up at his doorstep. They move in with him and work as secretaries for his agency.
Classifying Futakoi Alternative as a specific genre is extremely difficult, although at its core, it is most similar to a romantic comedy. The first handful of episodes are lighthearted, but the series becomes more serious as it approaches its conclusion. The criticisms about the show is normally the same as its accolades: Namely, that it never settles completely into any one genre or style and switches between them from episode to episode (being almost equal parts romance, action, scifi/fantasy and comedy).