Oliver Day

Oliver Day was a human who recovered from a bout of amnesia to discover that he 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. Oliver travelled to the scene of the explosion. In the rubble of the house, Oliver met the monster again, who threatened to kill him. Oliver was saved by the Doctor, who offered to let Oliver continue travelling with him. (ST: Attachments)