After the theme song ended, the first episode tells the story of the male protagonist being thrown overboard from a ship, ultimately washing up on the beach, and meeting the female lead.
This is a deserted place, the legendary Dead Island. They must return to civilization, but the male lead, washed ashore by the sea, has forgotten who he is, and even more so, he has forgotten his identity as an Assassin.
The female lead, Eva's identity, isn't revealed from the start, only a brief introduction that she arrived on this small island by accident and has been waiting for rescue, longing to return to human society.
There are many strange creatures on this isolated island, and Eva studies chemistry, which is why she has been able to barely survive on the island by defeating those lurking monsters with homemade chemical concoctions.
