Currently you are able to watch "Infernal Affairs" streaming on Criterion Channel. It is also possible to buy "Infernal Affairs" on Amazon Video, Apple TV, Vuduas download or rent it on Amazon Video, Apple TV, Vuduonline.
Chan Wing Yan, a young police officer, has been sent undercover as a mole in the local mafia. Lau Kin Ming, a young mafia member, infiltrates the police force. Years later, their older counterparts, Chen Wing Yan and Inspector Lau Kin Ming, respectively, race against time to expose the mole within their midst.