The game is afoot (or the game is on) is an idiomatic expression used to indicate that something interesting is about to start, or already has started.
Using the expression can relate to both literal and more abstract situations, making it a very versatile language tool.
You can say the game is afoot in a sports competition, when playing hide and seek, or before you are about to absolve a difficult scientific task. Also, this idiom can be used to accept a challenge.
The origin of the expression traces back to the great poet and writer William Shakespeare, who included the line “Before the game is afoot, thou still let’st slip.” in his 1597 play King Henry IV, Part 1.
The current meaning of the expression, however, is more tightly connected to the fictional detectives Sherlock Holmes and Watson.
The expression features in thousands of literary works, mostly within the realms of entertaining literature.
Online, the game is afoot appears on various image macro memes that show versions of Sherlock Holmes.