How we change what others think, feel, believe and do
The plot coupon is something that must be held or used in order that a critical conflict must be resolved.
There may be multiple plot coupons that must be collected before they can be assembled and used.
Plot coupons may seem like irrelevant MacGuffins until they are used for significant plot action.
To fight the monster, the hero must acquire a magical sword, shield and super-speed shoes.
A number of clues must be found and pieced together in order for the criminal to be detected.
The hero is given a grenade early on and carries it everywhere. Eventually it is needed to blow a hole through a wall to get out of sticky situation.
The Plot Coupon was first described by Nick Lowe in his 1986 essay The Well-Tempered Plot Device. The wit of the name derives from the idea that, as in a supermarket promotion, if you collect enough coupons you can exchange it for something of value. Lowe says of the plot of Black Star, by Lin Carter:
...I like to term this kind of thing Collect-the-Coupons plotting...So what you do instead is write into the scenario one or more Plot Coupons which happen to be "supernaturally" linked to the outcome of the larger action; and then all your character have to do is save up the tokens till it's time to cash them in.
For a less experienced writer, plot coupons may be used as substitutes for the more effective structural elements of a well-constructed story. As such they are effectively substitutes for plot.
Lowe, N. (1986). The Well-Tempered Plot Device, Ansible 46, July 1986.