A shipwreck leaves a pair of Spanish sisters and their schoolteacher alone on a remote island in the Philippines. The schoolteacher (Miss Muro) is played by a deliberately frumpy looking Lina Romay (with glasses, stockings, and bloomers to let you know she's playing against her sex-symbol type). While the three bathe in a lagoon, they find out they are not alone when their clothes are stolen. Lina keeps her bloomers and blouse, but the girls spend the rest of the movie clad in necklaces which sometimes cover them up top and island-fashioned thongs. The nudity isn't as creepy as it looks on paper -- the young sister (10-ish years old) is not sexualized, and the actress playing the older sister is clearly much older than her character (stated to be 14 in a flashback, but clearly closer to 24 in real life). They come to find that they share the island with a crazy old man who lives the bachelor life in a cave with the spoils of shipwrecks to keep him company.
The young sister is the clever one of the bunch (reading Kant for pleasure). The older sister experiences her first period and a sexual awakening that is aided by the appearance of a young fisherman from a nearby island. Upon witnessing the older girl's transformation, Lina begins to realize what she's missed out on through her solitary lifestyle.
Without giving too much away, a bit of an action plot does unfold at the end, giving this a little more substance than your standard Blue Lagoon knockoff of the era. It's no masterpiece, but it's certainly a cut above most of the throwaways Franco was producing in this era.
