Three irresponsible teenagers 'borrow' the 'seawitch' - a yacht belonging to the local seascouts and run out of fuel off the notoriously dangerous Bradda Head. In an endeavor to stop themselves from drifting on to the rocks they try to set the sails but they lose the mainsail overboard and it becomes entangled with a floating mine. The tide turns and the yacht drifts back towards the harbour towing the mine with it. Realising that the mine might hit a tanker anchored in the harbour the teenagers, aided by the scouts, make desperate attempts to alter the course of the mine until help arrives.
