Jack Hardy (David Jason) is the sole survivor of a mysterious submarine disaster in World War II. Now, in 1981, the ill-fated submarine Scorpion has resurfaced, miraculously intact. Naval Intelligence need Hardy's expertise to retrace the course and mission that led to her original disappearance. But what's waiting for them under the grey waters of the Baltic Sea turns out to be much worse than the Russian submarines they're expecting. Wherever Scorpion has been all this time, it's brought some very bad luck back with it...