Oliver Day

Oliver Day was a human who recovered from a bout of amnesia to discover that he had spent several months travelling with the Fourth Doctor.

Biography
Oliver's amnesia was caused by an incident with an alien creature and an explosion in a house. He travelled to the scene of the explosion. In the rubble, Oliver met the monster again. It threatened to kill him, but he was saved by the Doctor, who offered to let Oliver travel with him. (ST: Attachments)