In the first programme of the series, the team uncovers the world of the giants that reside in and on our western seas.
A few metres off the Cornish shore, the team study Britain's largest fish, the basking shark. Despite the fact that they grow up to ten metres long, little is known about them, which makes effective conservation very difficult. In an exceptional encounter, the team is surrounded by up to 12 sharks as they feed on microscopic plankton.
Their shark expedition then takes them north to the Isle of Man. Working alongside local scientists, they take shark DNA samples using a kitchen scourer in order to assess the genetic health and long-term fitness of these great leviathans. The team also runs into a giant swarm of jellyfish.
In the waters of South Wales, Tooni encounters an invading army of giant spiny spider crabs. These creatures boast a leg span of over a metre across, and Tooni reveals that they come into the shallow waters every year to find a mate.
On Lundy Island off the Devon coast, Frank assesses whether the island's protected underwater No Take Zone could be used as a template to establish a nationwide network of marine nature reserves right around our island.
Frank also reveals how Great Britain still relies on the sea to import goods. He boards one of the biggest transatlantic container ships in the world, The Atlantic Companion, as the vast ship brings its cargo into Liverpool Docks.
In treacherous waters off the Isles of Scilly, Paul dives the largest shipwreck in British waters to assess the legacy of the worst ecological disaster to affect our shores so far; the ill-fated Torrey Canyon oil tanker.