The citizens of Mercan cannot conceive of worlds beyond their own. 

Their sun, Mercaniad, is prone to deadly, radioactive flare-ups, and the Mercans have organized their life around the need to survive The Ordeal -- 

until a strange visitor appears from out of nowhere...

The Enterprise, badly crippled and in desperate need of repairs, must seek help from a people who cannot believe in its existence. 

Mercaniad is about to blow, and James Kirk faces an impossible choice: to attack the sun itself and save his ship and crew -- 

or let a people live in peace, in the only world they know...