Deep Star Six takes place on the ocean floor where a group of scientists and technicians are attempting to create the first underwater nuclear missile facility. When things being to go awry, the crew gets orders to pack up and go topside. Blocking their way is a creature that had been undisturbed for millions of years in a sea cavern but which has been released by the scientists meddling. The crew must get to the surface before the creature (or an imminent explosion) triggers their destruction.
It should be obvious by now that this plot has been rehashed quite a few times (Leviathan, The Abyss). The acting is average. Don't waste your time on this one.