A "foolproof plan" means a plan or strategy that is designed so well that it cannot fail, even if someone is not very smart or makes mistakes. Full definition
The only foolproof plan for the modern job hunter is to make sure to keep up to date with the newest advice and trends. (inspiringinterns.com)
Quitting their jobs is not an option, so — fueled by alcohol and dubious advice from a criminal (Jamie Foxx)-- the men devise a complex and seemingly foolproof plan to permanently rid themselves of their terrible bosses. (mrqe.com)
by Kam Williams For movies opening December 4, 2009 BIG BUDGET FILMS Armored (PG - 13 for intense violence, disturbing images and brief profanity) Action thriller about employees for an armored car company whose foolproof plan to steal $ 42 million from one of their own trucks goes horribly wrong when one of the conspirators (Columbus Short) has misgivings after a cop (Milo Ventimiglia) is shot during the heist. (kamwilliams.com)