The show is about Nigel Marven travelling back in time to visit the seven deadliest prehistoric seas. On his journey, he encounters many marine animals, from the predators to the prey. He travels to those time periods on a boat called The Ancient Mariner, a 24 metre long boat with a crew. His means of time travel is never shown but the closest thing to it is his time map, that show the seven deadliest seas and the creatures that live in them. On his dives, he wears a scuba set with a fullface mask so that he can narrate while under water. On some of the most dangerous dives, he uses a strong shark cage, which is spherical so that it is difficult for predators to bite it.