After Freya Parker engages detective Ken Feels to see if her husband is cheating, she fields a sext from husband Will Pounder and confronts him: "You're feeling frisky today." Pounder is surprised because he sent that sext to somebody else (Cherry Kiss, to be precise), which we find out when he repeats it to her, in person, in bed.

Parker, still suspicious, drops in to Pounder's office to join him for lunch but he comes in late (because he was banging Kiss) and needs to work through lunch because boss Ryan Driller needs Pounder's reports by day's end.

Over coffee, Parker confers with Driller's wife Alex Coal, who reassures Parker even as Coal starts to consider that Driller may be cheating too. Feels reports to Parker that he followed Pounder to an apartment and he may have found Pounder's paramour (Kiss), a "business consultant," adding that he thinks "Any man who would cheat on you has to be crazy."

When Kiss visits Pounder's office, she finds Driller there and shares her expertise with him, first in business matters, then in bed. Meanwhile, Parker beds Feels to even the score with Pounder ... but when he asks, "What's next?" as they're getting dressed, she aloofly dismisses him.

Things get clearer—and muddier—when Parker hands a wad of C-notes to Kiss, who reflects, "I've been hired for lots of kinky things, but this one tops them all," before Pounder and Parker clink wineglasses and toast the success of their little thrill game ... and Pounder heads off for a celebratory tryst with Coal as Parker busies herself with Driller. 

Game On is a caper movie with sex instead of money. The casual infidelity makes it a bad choice for couples unless they stay till the end and realize the game-playing aspect of the plot.