Use when someone can no longer continue their deceitful behavior because they're caught. Once the hidden cameras were found, everyone knew the game was up.
Pro Tip 2/3
Revelation Moment
Marks the moment when the truth is revealed, often after a period of suspicion. When the test answers were found in his pocket, the teacher declared the game was up.
Pro Tip 3/3
Caught in Action
Often used when someone is caught during the act, leading to an abrupt end of their schemes. She tried to sneak out quietly, but her mom appeared and said, the game is up.