Sun Worship (слэш)

The Our-I'Chi were a myth, similar to werewolves in human culture.
As a child Spock was bitten by a Our-I'Chi. Being not vulcan his form appears less cat like then the desert creature that bit him, he appears... more dog like. Luckily unlike werewolves Spock is slowly able to learn how to control his transformations that are controlled by the sun.
Spock is aboard the starship enterprise during the 5 year mission when he mysteriously changes form - and can't change back! He has never told anyone that he was a 'werewolf' so must choose to tell someone, or hide - and somehow find a way to transform back into a vulcan.
 
TOS because mention vulcan instead of new vulcan :P its tos now