A high ranking Government official makes a secret visit to the Endeavour to determine whether General Trust Corporation may use it's Minus Five equipment to commercial advantage without affecting the natural heritage of the Great Barrier Reef. The project is placed in jeopardy when, riding in the mini-submarine, he nearly loses his life. Dr. Paul Hanna, of the Government Science Council, joins the team on board the Endeavour.

An industrial spy gets aboard the Endeavour to photograph the top secret equipment, but he turns out to be not such a bad bloke when he realises his methods endanger Captain King's life.