Jake Olsen was an emergency medical technician who was killed as collateral damage in a battle between Thor and the Destroyer. Thor was killed in the same battle. The enigmatic Marnot restored them both to life by merging them. For a period the immortal Thor switched to the mortal Jake Olsen. Only the body of Jake actually survived and Thor had to adapt to the life and relationships of his host.
The spirit of Jake became a prize involved in several plots by Mephisto, Loki and Odin until allowed to have a final death.