From publisher blubr:
Being a hero is easy if you have laser vision, invulnerability, or super-strength. It’s a lot harder if you’re a bunch of nobodies with crappy powers, and you make up for that with a lot of heart—because by golly, you’re heroes too.
Third Stringers is a Backstory Cards Setting Grid to use during the Backstory Cards process. Taking turns going around the table, each player draws a Backstory Card and answers the prompt on the card, referring to the setting grid for random elements.
Think of it like a dynamic campaign or one-shot outline. The PCs are hero-aspirants that have your hearts in the right place, but your powers are, shall we say, underdeveloped. But you keep on heroing, because it’s the right thing to do. Who knows, maybe tonight will be your chance to shine!