I grew up on a diet of Poirot, Jonathan Creek and James Bond but much as I love the idea of him being a 'spy', I fear the reality may be a little more mundane. If you have a specific theory please feel free to share it in the comments below.
There are many theories relating to the Somerton Man: Who he was, What he was doing in Adelaide, Who killed him, and Why. Some of these theories are a little weird, and some are quite horrible (just read the comments section on the Smithsonian site.) All theories are welcome, but please remember that people are under no more obligation to agree with your theory, than you are to agree with theirs. I will also do my best to moderate any theories suggested so please keep it civil. If someone disagrees with your theory, this is not an attempt to shut the conversation down, quite the opposite, differences of opinion allow us to explore different possibilities and as such opens up the conversation. (So if someone wants to suggest that aliens are to blame they are free to do so, safety scissors are available.)
If the facts don't fit the theory, change the facts.
Einstein recognized the human tendency of allowing pride to overrule honesty. When the facts prove that our theory is wrong, the temptation is to change (or at least 'cherry pick') the facts so we can continue to have the admiration of our peers and cronies, or in some cases so we can acquire the necessary funds to continue our work (but don't worry if that fails you could always set up an Indiegogo page and get gullible Americans to fund you (or not as the case may be)).
In reality of course we really must make the theory fit the facts! We know very little, and it is incredible how much theory and speculation is now touted as fact. I have presented on this site the facts only as they were presented to me, if you feel I have made an error please let me know and I will do my best to correct it, but remember that there is as great a difference between correction and contradiction as there is between fact and theory.
Spy Theory 1 The Somerton Man was a spy working for the Soviets, he was discovered and assassinated by Australian forces.
Spy Theory 2 The Somerton Man was a Soviet spy who had decided to betray his country and start providing intelligence to the Australians (in return for asylum), he was then taken out by the Soviets. I don't think the Russians would have thought twice about killing someone on Australian soil.
Spy Theory 3 The Somerton Man was an Australian spying on the Soviets who was discovered and assassinated by the Soviets.
Spy Theory 4 The Somerton Man was an Australian secretly working for the Soviets who feared his cover had been blown and decided to commit suicide rather than face trial as a traitor and communist sympathiser.
Suicide A man, possibly already dying of a terminal illness, wishes to get dressed up one last time, look out on a fantastic Australian sunset and end his life.
Positional Asphyxiation It was an accident, the man had gone for lie down in the sun, fallen asleep, slid down the wall slightly and because of the position of his head and neck was unable to breath.
A Mother's Instincts The Somerton Man, Robin's biological father returns and tries to take the young boy away with him. Jestyn and/or Prosper murder him and together they dispose of the body on the beach after dark.
A Family Affair The Somerton Man was an abusive relative of Jestyn's, acting in self-defence, she kills him, and with help from Prosper disposes of the body on the beach after dark.
Body Swap We do not know for sure that the Somerton man is the same man that was seen in roughly the same spot the night before. Some people believe that they're different people, and that a dead body was placed on the same spot as another man had been lying.
Assassination by wildlife Australia has plenty of interesting wildlife, some of it very poisonous. Could the Somerton Man have been bitten or stung by something deadly?
Ballet Dancer Theory Was the Somerton Man a Ballet Dancer? Did Jestyn push Robin into ballet so he could follow in his (biological) father's footsteps?
Horse Doping Was the Somerton Man involved in illegal gambling and horse doping? Was he killed to keep him quiet, or because he messed up and a lot of bad people ended up losing a lot of money?
Forged Ration Coupons Was the Somerton Man involved in forging ration coupons? Did he try selling Prosper some forged petrol coupons? Did Prosper find out they were forged and kill the Somerton Man?
Natural Causes Whilst most people suspect foul play, and believe it to be murder. I would not be so quick to rule out natural causes.
Cover Ups and Planted Evidence Really the only thing left is to blame the illuminate or aliens, definitely the aliens.