[jacky43]

Infernal Affairs is a 2002 Hong Kong crime-thriller film directed by Andrew Lau and Alan Mak. It tells the story of a police officer who infiltrates a triad, and a police officer secretly working for the same gang. The Chinese title means "The Unceasing Path", a reference to Avici, the lowest level of Hell in Buddhism, where one endures suffering incessantly.

The English title is a word play, combining the law enforcement term "internal affairs" – typically the division of any law enforcement agency that would be responsible for (among other things) finding a mole – with the adjective "infernal", meant in this case as a reference not to fires or infernos in general, but specifically to the inferno of Hell ("Inferno" being the Italian word for "Hell").

Thus, the English title is both a phonetic pun and – like the Chinese title – an allusion to a place or condition of eternal suffering. Infernal Affairs is the first in the Infernal Affairs series and was followed by Infernal Affairs II and Infernal Affairs III.
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It took me 15 yrs to understand Anthony Wong (the officer in charged of this ops), telling Eric Tsang (underworld boss) a joke on kidney. It goes something like this.

2 men need one kidney but there are only one. A man took out a deck of poker cards shuttle it and each pick one, w/o seeing put it in his pocket. They guess what is the correct card in the other's pocket.

The winner live and the loser dies.

Morale of the story. Both guess wrong and die. Anthony Wong never mend it a joke but a warning to Eric Tsang. The police will arrest him one way or the other.