On 31 January 2014 NBC carried a report about a man who had washed ashore on the remote Pacific island of Ebon Atoll, having spent a year lost at sea surviving on turtle blood and raw fish. In the days that followed the world's media was enthralled by Salvador Alvarenga's tale, as it emerged that he had drifted 6,700 miles from where he left the south coast of Mexico on a routine fishing trip. Some thought the real-life Robinson Crusoe story too far-fetched, and the castaway's own reticence in speaking to journalists did little to quell his doubters. Now, having interviewed Alvarenga over 40 times, Jonathan Franklin is seeking to have the last word on this remarkable tale with his book 438 Days. Read an edited extract on the Guardian website.