Tim Dunn and Siddy Holloway explore the labyrinthine Camden Town station, and the forgotten wartime shelter built beneath. Plus, Siddy visits a station which hides a lost river.

Tim Dunn and Siddy Holloway explore the disused areas of South Kensington station, with platforms reclaimed by nature and wartime uses. Siddy also heads to Marylebone to reveal the original features on the platforms and the tube infrastructure hidden within the walls of a hotel.

Tim and Siddy are heading to a station you won’t find on modern-day tube maps – Dover Street, now known as Green Park. During the Second World War the abandoned passageways and lift shafts of the original station had an incredible second life as the base for the London Passenger Transport Board whose work kept London’s transport moving against all odds. Next, Siddy delves into the abandoned corridors and platforms of Down Street, closed to the public in 1932, and the scene of some of the most pivotal decisions of World War II.

Tim and Siddy embark on a night-time track walk to the abandoned station of British Museum. The pair discover enormous 1930s hand-painted adverts and evocative white tiling. During the second world war it served as a shelter and spine-tingling clues to the children who stayed there during The Blitz can still be seen.

Siddy visits the elegant, white-stuccoed houses of Leinster Gardens, which are hiding a stunning secret of the London Underground behind their walls. These aren't all real houses at all, they are in fact, an incredible quirk of the Underground's history of steam.

Tube fans would be right in thinking there are 272 Underground stations on the network, but Siddy Holloway has such unique access, today she is taking Tim Dunn to the 273rd. This station only has a west bound platform, no customers and no members of the public will ever board its trains. Situated on the 3rd floor of an unassuming tower block in west London, this is TFL's state of the art training facility.

Siddy also heads to one of the oldest and friendliest deep level stations at Oval. She reveals unusual brick lift shafts, surprising ventilation and the international phenomena that started life on a simple station white board.

At the depot Tim explores the wild world of experimental and innovative trains and has a mind-blowing experience when he samples hot sauce, homegrown on a tube station platform.

Tim and Siddy are exploring the station in the beating heart of London's theatreland - Leicester Square. It's a place adored by tourists and culture lovers but very few know of the secrets hidden below ground. Siddy reveals the extraordinary previous life of the station office - as a display cabinet for V&A antiquities. The pair then ride what was once the world's longest escalator at 54m, explore layers of the stations design history hidden in unassuming cupboards and see the unique wartime communication infrastructure still stored in abandoned lift shafts. Tim learns more about the work to preserve the networks heritage from TFL's Ann Gavaghan.

Siddy visits the tourist hot spot of Hyde Park Corner, where the stunning Oxblood Leslie Green station building has had a renaissance as a high-end hotel. She delves into its abandoned cross passages, adorned by stunning original tiles and reveals enormous fans and gloriously aging stair shafts.

Tim Dunn and Siddy Holloway pack their bags to go on a trip around the sprawling Heathrow Airport underground stations - the first ever underground rail link between an airport and a city. They start their trip at the original Heathrow Central, now Heathrow Terminals 2 and 3, stopping off at Hatton Cross, with its eye-popping 70's mosaic tiling and speedbird logos, before they finally reach the futuristic 80's design of Terminal 4. Finally, they embark on a special trip along the Heathrow loop, the tube tunnel link which passes underneath the runway. Climbing through the driver's cab, they alight at a secret platform to explore a ventilation shaft and emerge above ground to the sound of aircraft.

Next, Siddy heads to north west London to delve into Swiss Cottage station, one of the first stations to be built on the expanding Metropolitan Railway. With those original platforms closed to the public 80 years ago, she explores what remains of that forgotten world.

Shepherd's Bush is a fabulous example of how the network has changed and adapted over time and Siddy Holloway knows where all the best bits are, ready to show Tim Dunn. Just off the platforms the pair discover Victorian glass tiles, long abandoned passenger tunnels from the original Central London Line, epic vents with an eye-level view of people on the platforms and a lift shaft with an escalator through the middle. They visit a gigantic cavern hidden within the body of the station and the perfect example of how nature sometimes beats the best laid engineering plans.

Siddy delves into Elephant & Castle, a small station that packs a big punch. She discovers what it takes to be at the helm of a train from driver Jennifer, reveals the only original 1890 tiles still visible to the public and the spooky discoveries hidden behind a platform door.

Tim Dunn and Siddy Holloway explore the sprawling TFL maintenance facility - Acton Works. Staying in Acton, Siddy reveals an oddity of Underground history.