Whilst observing a Russian ship, HMS Hero finds and takes aboard an experimental unexploded mine. With the mine onboard Captain Holt makes plans to sail for Belfast. However his chopper pilot, on a routine flyover of an uninhabited island, spots a man there. The MoD orders Hero back to the island while bomb disposal expert Lieutenant Commander Napier looks after the bomb. Meanwhile a search of the island is conducted to ascertain who if anyone is currently living there. Their search eventually yields the secretive Mr Robertson.