OK, so a brief synopsis..
You play as Leaf, bodyguard to princess Marika (both are side characters from Lily Knight Saga; most Tsukinomizu games are part of a larger shared universe). There is an island full of ruins and magical artifacts that are currently being administered by succubi under a treaty with the kingdom. Marika was tasked with inspecting the administration of the island when the ship had mechanical issues, forcing them to stay on the island longer than expected. Unbeknownst to Marika prior to her arrival, the succubi are using the island as a (lewd) tourist attraction, and during their stay, Marika gets kidnapped and put on (again, lewd) display. It's up to Leaf to rescue her, but in doing so, she must navigate the various zones of the island which are populated with lecherous monsters and convenient ecchi traps (pitfalls). All the while, the succubi on the island plot to make Leaf's ordeal.. "more interesting" for the tourists.