IoIo Williams continues his journey around Wales, visiting the harbour of Trefor on the north Llyn coast, an area renowned for its sea kayaking. The presenter learns about a 150-year-old shipwreck near Nefyn, where the slate cargo has been transformed into a reef brimming with sea life, sets sail on a fast offshore racing yacht from Pwllheli marina, and discovers the nocturnal underwater world lying just off the coast from Criccieth.