While investigating the disappearance of research ships in the Aegean, the Seaview is damaged by a bomb. Captain Crane goes ashore, and pretends to be a diver for hire. The Captain meets an ex-diver, now a mute due to a tragic accident, who claims to have seen a "city beneath the sea". Crane gets hired by the "bad guy", but his cover is soon blown. He is taken prisoner and nearly loses his life.