You hear a rumor (from two chatting NPC's) that there has been recently a crime-spree by a burglar, steeling stuff from various people. The first quest update tells you who the last victim was. When you talk to him, he tells you that the burglary was very odd as the burglar stole only a few, relatively worthless items, and didn't even seem to notice the much more valuable goods such as the silverware. He also tells you that he asked around and talked to the other victims, and they tell the same story. He also tells you that one of the victims' stories is unbelievably strange, and that you should talk to him about it personally. When you do, the second victim tells you that the burglary actually occured when all the family members except him were in the house, and they didn't notice anything because the burglar looked and acted exactly as the victim. Noone even suspected anything until the real deal showed up and asked where the stolen items were, only to be dumbstruck by the answer that he himself had taken them! After that, you ask around the local guards if they happened to recall where "the victim" went and what he was doing. Eventually one of the guards recalls a very strange event when he saw the impostor walk into an alley and then heard him yell in pain. He rushed to his side... only to find... or rather not to find him at all! In the alley there was exactly one man - a completely different man! When the guard asked about where the other one disappeared, how he got there and what was the yelling all about, all the man did was stare at him in confusion... He had no idea what he was talking about. After a while, the guard let him go, but watched where he went. He gives you the location of a small house on the edge of town. The door is locked, so you have to use a lockpick. Once inside you don't find anything that looks suspicious... until you check on the desk and find some very strange notes. The first four notes appear to be written by four completely different people, with the only thing in common about them the fact that they are all about how those people woke up in a place they did not recognize, unable to remember how they got there or what they were doing before, carrying items they didn't recognize. The only reason why those notes ended up in that house was because all four people found that they had a key with the address written on it, so they went to find an explenation, but found noone inside. So they left a note for the owner, describing their predicament and saying that they went to look for answers elsewhere and would be back at a later time. But then you check the bottom-most note... and discover something interesting...
Apparently, the house belonged to a paranoid mage who believed that he was being watched, but couldn't figure out by whom. Eventually he reached a point where the suspense was too much to take, so he devised a spell that would let him read minds. The spell was difficult to devise, but eventually he stumbled upon the idea of temporarily copying a person's personality (with all their thoughts and emotions included) into himself to have it as a sort of "book" handy to discover any secrets that the person kept only to itself. He would use the spell secretly to try to locate his stalkers and take action against them. The note ends on his preperations for the first experiment with space left to document the results. But that space stayed blank. Feeling that this is a clue to what's really going on, you bring the note to a specialist in the local Mage's guild. He takes time to study the mechanics of the devised spell carefully, but the next day he reveals an astonishing find. According to his calculations, the spell is both a success and a filure at the same time. Indeed, it would let you copy someone else's personality, but there was a very nasty side effect: the personality of the user of the spell would be completely overshadowed by the newly gained one, making said user think that they actually are the person whose personality they stole! And that would not only affect the mind, but also the body, making the user nigh exact copy of the target of the spell! Putting you're heads together, you speculate along with the specialist that the paranoid mage, thinking that he was someone else, would unknowingly get into people's houses, thinking that he was them, and upon leaving would take what he, or rather the person whose personality he had stolen, would consider neccessary for whatever he had in mind, resulting in unintentional thefts. Since he never wrote any other note as himself, the enchantment must have gone unstable and uncontrollable, turning the mage into a confused, wandering doppelganger, unwillingly stealing the identity of people he would run into. Fortunately, this instability meant that locating him by magical means would be failry easy, as the flux of his magicka would clearly stand out against the stability of his surroundings. You use a special modified Detect Soul spell to track him down in the nearby forest, this time taking on the identity of a hunter. When he sees you, the worst happens... the spell triggers once again, and he copies the closest person to him... you...
After that, unsurprisingly, an identity argument ensues. Both you and him argue who is who and you try to convince him about what's really going on. Depending on your persuasive ability, there are two possible outcomes:
1. He gets into a fit of rage and a fight ensues (a tough one, since he has all of your abilities and strengths, including the dragon shouts!!!) you can simply kill him, or you can try (according to the suggestion in the very last quest update) to bring down his Stamina very close to zero, at which point the exhaustion will cause the spell to lose power and disperse.
2. You manage to convince him that he was an unwilling impostor, and he says that a Dispell spell could do the trick. If you already have the spell, you just use it. If not, he follows you as a companion (still a copy of yourself, but watch out! get into a fight with another NPC and he will turn into him, and then you'll have no other choice but to kill him (or bring his Stamina down) until you can get a Dispell spell from the specialist.
If in either scenario the paranoid mage is still alive and turned back into himself, he offers to return all of the stolen goods and throws in an enchanted ring that has a permanent Detect Soul spell that he claims gave him some confidence that whoever was watching him couldn't catch him off guard. Then you can choose to:
1. Return the stolen items to their rightfull owners without explaining where and how you found them (i.e. show mercy for the mage), in which case you'll only get to keep the ring, but your reputation will benefit.
2. Keep quiet about the story and keep all the goods for yourself.
3. Refuse the offer and instead turn him in to the guard (him having the stolen items on him is undeniable evidence). You'll get rewarded for solving the case with gold, but the word spreads of how you didn't show mercy for the poor soul, so your reputation is left unchanged.
If you simply kill him during the encounter you will find only the stolen goods, which you can either keep or return.