The sinking of the Titanic in April 1912 was a tragedy unlike any other. Surely it could never happen again? But it did. Incredibly, Titanic had a sister - Britannic - who suffered a near-identical fate. In the wake of the Titanic disaster, Britannic was re-engineered to be even stronger. And yet, on 21 November 1916, she sank in just 55 minutes - three times faster than Titanic. It's one of Britain's greatest untold disaster stories. Now on the 100th anniversary, presenters Kate Humble and Andy Torbet piece together exactly what happened in those 55 minutes. While Andy makes a dangerous dive to the wreck, Kate speaks to descendants of the survivors, tracked down for the very first time on TV. The characters she uncovers include Violet 'Miss Unsinkable' Jessops, who survived both Titanic and Britannic, Captain 'Iceberg Charlie' Bartlett and look-out Archie Jewell, who miraculously survived when those around him died. Using rarely seen and unpublished diaries and letters of the captain and crew, Titanic's Tragic Twin recreates what it was like for Britannic's survivors to fight for their lives and investigates how and why Britannic's victims died horribly - and avoidably.