Old book.

Martha and Ted were best friends and work colleague but never found each other attractive it was just simply business between them both.

That is until Martha accidentally swapped their bodies & she did not know how it happened.

One minute she was sat in the library reading an old book about black magic in the United States and in the next she was in teds flat.

She immediately called her own cell phone and told Ted to come home.

Ted was frightened to be walking home in her female body.

There was one part of the spell she did not know about they would be immediately attracted to each other and would not be able to keep their hands to themselves.

When Ted reached his own flat and saw his own body for the first time through her eyes he felt incredibly horny and could feel the void between his legs getting wet and his nipples harden with anticipation. 

That's how Ted found himself on his back telling his former body just slide my knickers to one side and put it in.

Ceiling them both in their new bodies for life.

But you do not hear Martha and Ted complaining about that.