Mujintou Monogatari X ~Gaiden~

The story behind Desert Island is that you and six other people (all girls) have been anesthetized and brought to an uninhabited island. You woke up from theanesthetic and found you and your sister is on an uninhabited island somewhere in the Pacific. You and your sister have no clue on how you two got on the island. However, you have soon found a radio inside a hut on the beach. From the radio, you are given the information that you and six other people have been transported to this island under the experiment called "J Project." The company conducting this experiment has informed you that you will have three-month to uncover the secret of J Project. From here on you will begin your exploration of the island to uncover the secret of J Project and gather up girls.

[From Anime Densetsu]