Supercar is to be tested underwater to see what depths it can reach. With Bill Gibson and Beaker on the shore, Mike takes the craft 400 feet down to the seabed. suddenly, the cockpit leaks and the engines are affected. Beaker tells Mike what he must do, and soon he has restored power, only to be caught up in a mine cable. Bill Gibson dons his diving suit and goes down to try and free Supercar before the mine explodes. The tests prove, ultimately, to be a success.