Leah Thomas. The Ex-Corporal. (Asimovs August 2013)

A little gem of a story from Thomas.

A young girl’s father develops epilepsy – and being a bit of an sf buff, he passes off his minutes spent in seizure as time spend in galaxies far, far away. But this fiction appears to become reality, chillingly so as his personality changes, and someone else, far less pleasant, appears to be taking over his body.

It’s only a few pages long, but grabs you and doesn’t let go.