In which Desmond tries to stay in the Animus as long as possible - the machine being his only connection to the outside world. He was kidnapped, and is held captive - that wouldn't be out of question. A little power surge (our outage, due to whatever reason) just helps him to do it completely - he ends up in the past... Sans his body
+1 if he can only interact with others when he possesses some poor unsuspecting soul
+1 if the body he ends up occupying is female
+1 if he gets to keep his assassin skills
And, finally, +1 if Ezio/Altair/Leonardo/Malik ends up stalking him