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.

In the second programme in the series, explorer Paul Rose, marine biologist Tooni Mahto and maritime journalist Frank Pope explore the wild seas around Scotland. The team travel to Bass Rock, one of the largest gannet colonies in the world. They are there to try and find out why when most British seabird populations are in decline, the northern gannet is bucking the trend. Tooni helps scientists who are using GPS trackers to discover the extraordinary distances gannets can fly in the search for food. Paul goes beneath the waves to witness the amazing diving ability of Britain's largest seabird. Tooni joins a scientist in St Andrews Bay in search of the bottlenose dolphin to find out why dolphins have unique signature whistles - could they be names as we know them? She also takes Paul on a spectacular night dive at St Abbs in search of the amazing sea creatures that fluoresce beyond our visual spectrum. And intriguingly, this discovery has been used to help study cancerous cells. Expedition leader, Paul Rose, meets the hidden heroes of the Royal Navy. They are on a special clearance mission around Garvie Island at Cape Wrath. The Navy divers go underwater clearing live 1,000lb unexploded bombs dropped by aircraft during training exercises. These are modern and extremely powerful weapons that must be detonated with explosives - underwater. We have over 25,000 wrecks around the British Isles. These wrecks are a rich archaeological record of our maritime heritage, but sadly they are being damaged by trawlers, souvenir hunters and the forces of nature. Frank and Paul Rose go to the Sound of Mull, to see first-hand what can be done to preserve the history locked away in these relics.