Dorian's portrait appears to age, whilst he remains youthful as if he was frozen in time (like a painting). Dorian hides the portrait in his loft to hide this secret. Suspecting that there seems to be a connection, Lord Henry Wotten (Colin Firth) makes a copy of the key and enters the room to discover that the painting has aged beyond recognition. Dorian arrives at this time only to watch Lord Henry burn the portrait. Lord Henry Wotten's motivation in doing this is to protect his daughter Emily, who has fallen in love with the young Dorian Gray.
He is locked in the burning room by Lord Henry and the spirit within the painting appears to swap place with Dorian turning him old and dying in the process. The picture returns to it's original state as a young Doran Gray.
Thanks Rhett Fox!