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.

This time on Britain's Secret Seas, the team set out to explore the waters off our eastern coastline, which are constantly pounded by the raw power of the waves. These are the shallowest waters off our shores but over the centuries they have been some of our most productive, providing oil and gas as well as fish for our tables. But things are changing in the North Sea and our team discover a sea full of surprises. On the Farne Islands, they come face to face with England's largest colony of grey seals to find out what makes them such extraordinary divers and successful hunters. Grey seals can dive for over forty minutes on a single breath - the team find out how they do it and why they seem to be coping with the changing conditions on the east coast far better than the harbour seal. Elsewhere, Frank looks at the newest form of energy in the North Sea, at a wind farm off the Norfolk coast, and Tooni examines what effect the wind turbines are having on marine life. Paul uncovers the intriguing history of light vessels among the sandbanks of the Thames Estuary; and Tooni gets to the bottom of a bumper lobster harvest in the port of Bridlington. Paul dives with a shoal of fish that soon transform into a school when he starts to behave as if he were a predator. To find out how they are able to achieve such synchronised motion and why they suddenly change direction, he joins a scientist who has created a plastic imposter - Robofish.

In the final programme of the series, explorer Paul Rose, marine biologist Tooni Mahto and underwater archaeologist Frank Pope explore our busy southern shores. As one of the world's busiest shipping lanes, our southern seas are littered with wrecks. Outside Poole harbour, the team join an archaeological excavation of a mysterious 17th century ship. Bearing incredibly ornate carvings of a merman, Frank reveals this is one of the most important underwater discoveries since the Mary Rose. Paul joins the dive team working on the wreck to try to discover where this ship came from. But it's a race against time as the timbers are being destroyed; Tooni wants to find out why. Frank and Tooni examine how we balance the needs of marine life with those of people when the two come into conflict. Studland Bay in Dorset is home to a thriving population of spiny seahorses living amongst the rare eelgrass beds. Only identified as breeding in the area seven years ago, the intensive use of the bay, boat moorings and pleasure boats' anchors are threatening the seahorses fragile eelgrass habitat. Paul dives two wrecks in the search of the mighty conger eel to find out if, when he comes face to face with one, these incredible beasts will live up to their fearsome reputation. As the tourists hit the roller coaster and dodgems on Brighton pier, they know little of the wonder beneath them. Tooni dives underneath Brighton pier to reveal one of the best places on the south coast to see a great variety of our native marine life. Out in Lyme Bay, Frank dives the wreck of an experimental WWII submarine.