Dermot O’Leary uncovers the story of the RNLI’s heroic efforts in the Second World War, as its crews were called to serve in the most perilous of conditions, saving 6,376 lives.

In the first episode, a retired nursery teacher gets herself into danger when attempting to rescue her two dogs after they became cut off by the rising tide. In Wales, one volunteer crew member is washed off a cliff into the sea trying to rescue a concussed spearfisherman, and, in Blackpool, a rescue in a force seven gale goes badly wrong, putting all three crew members' lives in jeopardy. When torrential floods hit Cumbria shortly before Christmas the volunteers are again on hand to try and rescue families trapped by the rising waters.

The volunteers are called to rescue two students who have jumped naked into the River Thames while high on LSD.

The volunteers are called to rescue the crew of a fishing boat that has run aground in the middle of a storm.

The volunteers are called to rescue a father who gets himself into difficulties after trying to rescue his son.

In the first episode, a father and daughter find themselves treading water for almost an hour, hundreds of metres out at sea, after being swept out by a rip current. In Kinsale, Ireland, three fishermen swim for their lives as their trawler is smashed on to rocks. And in London, an unlucky tourist is minutes from hypothermia after his attempt to take a selfie lands him in the Thames.

Off the coast of Bridlington, a kayaker has been missing for an hour, and only the crews' expert coastal knowledge can help track him down. A crew member on his first shout races to a dinghy capsize to find a man losing consciousness in the water. And when a fishing trawler sinks off the coast of the Shetland Islands, the fishermen and RNLI crew aboard must swim for their lives.

In Ireland, the Castletownbere crew race to rescue a lone yachtsman who has lost his mast and is drifting powerless in 10-metre waves, 45 miles out to sea. In Minehead a woman on a clifftop walk has gone missing, and after 17 hours the crew are called to search the shoreline. And for an injured horse rider on a remote beach, the Porthcawl crew are the only way to emergency medical help.

In Bangor, Northern Ireland, crews are tasked to find two teenage paddle boarders, blown offshore towards nearby shipping lanes. The Tenby crew in West Wales must race to a dog who's fallen over a cliff - before worried members of the public attempt to rescue it themselves. And on the North Norfolk coast, the harrowing hovercraft rescue of three young sisters still brings back painful memories for the Hunstanton crew.

Every day around the UK, an army of unpaid volunteers put their lives on the line to try and save complete strangers. Saving Lives at Sea tells the story of the ordinary men and women of the Royal National Lifeboat Institution (RNLI) who, across the country, are ready to launch their boats and race to the rescue within minutes of a cry for help - 24 hours a day, 365 days a year, whatever the weather. Using footage shot on crews' own cameras, this series takes us right into the heart of the action, capturing the unpredictable work of the RNLI in never-before-seen detail. In south west Wales, the Tenby crew race to a kayaker with suspected back and neck injuries after being caught out by a freak wave. Neighbouring Cornish crews must join forces in an epic 11-hour battle to stop a 3,600-tonne coaster breaking up on the rocks. And the Dart and Salcombe crews are astonished to be paged to the biggest shout of their lives.

In episode six of this 12-part series, on one of the hottest days of the year, the Dover crew race to a Channel Swimmer who's collapsed within sight of the French coast. In Mudeford, there are growing fears for a dinghy sailor who is four miles out in rough seas and refusing to accept help, and the Tenby crew are called to a runaway yacht just metres from running aground.

When a climber falls in a remote cave on the Gower Peninsula, the Mumbles crew are called in to search and rescue. In Portreath, lifeguards battle to save a teenager who is determined to surf despite red flag warnings. And on the Isle of Wight, the Yarmouth crew must locate a man in the water amongst over a thousand boats competing in the Round the Island Race.

In Ilfracombe, a father and son find themselves in danger after jumping in to rescue a drowning dog. The Minehead crew must scour the cliffs and shoreline after a young woman goes missing in a distressed state. And when they're paged to a man seen entering the Thames, the Tower crew face the possibility they may not find him in time.

The Aberystwyth crew must improvise when a local paraglider crash-lands at the bottom of steep cliffs. When an adventurous seal is discovered on the steps of a block of flats, Redcar's newest recruit is first on the scene. And in Minehead, a challenging rescue forces an inexperienced crew member to consider her future.

As the summer sun comes out, so do the fun seekers, and crews around the country get ready for action. In Southend, a family of five out sightseeing are a mile off shore and waist-deep in water after being caught out by the tide. The Shoreham crew are paged in the early hours after a drunken festival-goer jumps in the river. And the Brighton crew fear the worst when a man tombstones off the pier at low tide.

When an early morning dog walker spots a flare off shore, the Rhyl crew race to one of the most mysterious shouts they have seen. The Lowestoft crew find themselves paged to help to a sailor who doesn't want to be rescued. And at Tower, a good samaritan is clinging on for his life after trying to rescue a dog which has jumped into the Thames.

In Tenby, police searching for a boat thief call for lifeboat assistance. The Mumbles lifeguards and lifeboat crew race to help two kayakers adrift in a boat laden with beer. And when two yachts capsize off Portsmouth, the crew must make fast decisions about who to rescue first.

Out at sea, minutes and even seconds can make the difference between life and death. On the south coast of Devon, the crew of Salcombe lifeboat station are called to two of the highest level of emergencies at sea - a mayday call to a fisherman pulled to the bottom of the sea ensnared in his own fishing gear and a devastating fire on a boat 15 miles out in the Channel. Over 200 miles away in the waters off Anglesey, as they go to the rescue of a father and his 13-year-old son missing at sea, the crew of Moelfre station uncover a story of survival and heroism.

From first timers at sea to salty old skippers, the wrong conditions can catch anyone off their guard leaving them in need of rescuing. On the east coast of Ireland, the crew of Courtown's inshore lifeboat face one of their most challenging shouts ever - in deep water they must keep a teenage girl with suspected spinal injuries immobile and afloat for 40 minutes, knowing any movement could leave her paralysed for life. Meanwhile on the English Riviera in Devon, the Torbay crew have just minutes to find a capsized kayaker before she is dashed against the rocks, and in Dartmouth, an 81-year-old sailor, fallen overboard in the middle of the night, is left clinging to a rope for his life.

Keeping watch over the River Thames, the crew at Tower in central London, the busiest of the RNLI's 238 stations, are called to a man reported in the water eight miles downstream. On arrival, they discover a river bus already on the scene and their crew desperately trying to hang on to the unmoving man before he disappears under the water. At one of the most remote stations, Trearddur Bay on Anglesey, the crew are called to an inaccessible coastal fishing spot, where an angler has plunged face first down a cliff onto rocks. With no helicopter available, the crew and their small inshore lifeboat are the only option to get him to urgent medical treatment. While in Portsmouth, gale force winds and a churning sea make a routine rescue particularly challenging.

In Brighton the crew are called out to what they think is just a routine job - towing a yacht that has lost power back to harbour - but a sudden spring squall and mountainous waves leave the crew fighting to save the yacht and themselves from being dashed on the shore. Off the east coast of Ireland, the crew of the small fishing village of Howth respond to an emergency call from a family on a local beach - a teenage girl out on a sailing dinghy has capsized and is now being swept out into the Irish Sea. While on the island of Anglesey, the crew of Moelfre are also heading out to help a family in trouble - two women have swum out to save a friend's young son being blown out to sea on an inflatable, but it quickly becomes the rescuers that need saving themselves

In Torbay in Devon the seas are often crowded with sailing craft of all shapes and sizes – when two collide the Torbay lifeboat crew are called out to provide emergency medical assistance for a potentially life-changing injury. On the south coast of Wales, the Porthcawl crew have just seconds to spare when two teenagers on holiday get caught on a rocky outcrop by a large swell and a rising spring tide. While in Yorkshire, the Flamborough crew have to pluck a man from danger after he jumps off a hundred-foot cliff for a dare.

There are many reasons why the thousands of yellow-wellied volunteers first joined up, but they all share a common desire to help those in trouble. At Anstruther station in Scotland, one of their newest helms, George, takes charge for his first shout to a serious medical evacuation. A woman has slipped on one of the most notorious parts of the station's patch, the Chain Walk, where walkers have to negotiate steep cliffs while hanging on to chains hammered into the rock. On the north coast of Devon, the crew at Appledore station make a life-or-death dash to rescue two teenagers caught in a cave being flooded by the tide and pounded by waves, but arriving at the scene they discover that saving the young couple will mean putting themselves in danger. And when Storm Henry hits the British Isles and has shipping running for safe harbours, a cargo ship loses all power and is left at the mercy of the waves and 90-mile-an-hour winds. As she drifts inexorably towards the rugged cliffs of north Devon, two lifeboats are launched but even with their combined power they face a losing battle trying to tow the huge ship out of harm's way

On the north coast of Kent, new crew member Ruth is on board for one of her first ever shouts. Off the Essex coast, the crew of West Mersea station launch to reports of a life raft in the North Sea. 10 miles along the coast at Clacton the crew get a call in the middle of the night to look for two people in the sea.

For the thousands of volunteers who guard our seas and waterways, saving other people can mean making sacrifices in their own lives. Volunteer and doctor Adam makes a 500-mile round trip every time he puts in a shift at Tower Lifeboat Station in the heart of London, but his medical skills prove invaluable when the crew are called to help the police try and save a man who is stuck under a pier on the Thames with the tide rising. In north Wales, when police officer Vinny isn't patrolling the streets of Rhyl, he volunteers for the lifeboat crew. When the pagers go off one summer evening, Vinny finds himself heading out into the darkness to try and rescue two teenage boys who have chased a wayward football into the sea and get caught in a treacherous current. While across the Irish Sea in Dunmore East, the lifeboat crew's evening is interrupted by an urgent call to a motor boat stranded several miles out and taking on water.