October 21, 1449 - February 18, 1478: Age 28
George Plantagenet seems to have been a rather ordinary man, with ordinary weaknesses, plunged into an extraordinary situation — that is, the dynastic mess already referred to on these pages recently with his great-great-great uncle, Richard II, and his neice, Elizabeth of York. Seems like February was a bad time to be royalty in Britain in the 15th century.
George Plantagenet, a.k.a. the Duke of Clarence, was the younger brother of Edward IV, the guy who deposed Henry VI, who was the grandson of the guy who deposed Richard II. In fact the only person who got the throne by direct inheritance during the 15th century who wasn't deposed and/or murdered was Henry V, who died of dysentery 9 years after he became king. And of the seven monarchs who ruled Britain in that century, a majority — Henry IV, Edward IV, Richard III, and Henry VII — got the throne by means other than conventional inheritance. (Given the quality of life of those who had it... why?)
Back to George. It's very simple really, he got embroiled in some kind of plot against his brother Edward. Edward had him imprisoned and condemned to death. Legend has it that he was given his choice of execution methods, and he famously chose to be drowned in a butt of malmsey, that is, a giant keg of wine. Apparently he was fond of the stuff. It sounds like urban legend, but Shakespeare knew a good story when he saw one (as do I) and wrote it into Richard III. In fact Plantagenet's body was exhumed many years later and although not even the CSI guys can prove drowning from a skeleton, it was clear he had not been beheaded, the usual method of execution for those of noble birth.
By the way, I have a little souvenir booklet purchased in Britain many years ago that shows all the lines of succession of Britain's monarchs. I have had to refer to it so frequently in this past two weeks that it now practically falls open at the page showing the line between Edward III (Richard II's grandfather) and Henry VII.
Sources: Wikipedia; Montague-Smith, Patrick, The Royal Line of Succession