Amnesia has been a great literary prop for centuries. One of the greatest action characters of our time had amnesia: Jason Bourne. But he is not the only one. In See Jane Run by Joy Fielding the main character has amnesia. The same is true of Before I Go To Sleep by S.J. Watson.

However this prompt does not require that your main character has amnesia, but just that one of your characters does. It has to affect the way the story proceeds in one way or another.

The other decision you have to make is what kind of amnesia your main character has. The two main kinds are Anterograde amnesia and retrograde amnesia. Anterograde amnesia is an inability to create new memories after the event that caused the amnesia. Long term memories are intact but nothing from the recent pass.

Retrograde amnesia is where most memories created prior to the event are lost, but new memories after the event remain intact.  Both can occur at the same time in the same person.

This is a fun exercise because you get to play with unusual parameters, but the details, the plot, the point of view, and the kind of amnesia are all up to you.