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.

The Endeavor's crew is ordered to determine if the wreck of a 18th Century galleon proves that the Spanish discovered Australia before Captain Cook.

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.