Jonathan McQuarry (EWAN McGREGOR) is a meek accountant who works as an audit manager, spending short amounts of time at various high profile firms doing the books and, in his own words, watching the world pass him by. Thus, he's drawn to Wyatt Bose (HUGH JACKMAN), a handsome and polished lawyer who befriends him late one night when both are working well past closing time.
When Jonathan ends up with Wyatt's phone and vice-versa, and Wyatt says he's been called to London for business, the accountant gets a peek inside his new friend's life. That's particularly true regarding a sex club where members call others for an evening of casual and anonymous sex, no strings or commitment attached.
Initially reluctant to participate, Jonathan soon becomes a willing member, enjoying a number of liaisons with various women. But when he meets a woman (MICHELLE WILLIAMS) he previously ran into on the subway, he wants to take things slower with her. Since all members are to remain anonymous, he only knows her by her first initial, S, and the two eventually end up in a hotel room.
His world then turns upside down, however, when he returns from getting some ice to find blood in the room and an assailant who knocks him out. When he comes to, S is gone, as is the blood, prompting police Det. Russo (LISA GAY HAMILTON) to doubt his story.
Upset and confused about what's happened, Jonathan tries to find S, only to discover that she's been kidnapped and he's being blackmailed in exchange for her well-being. From that point on, the accountant sets out to find out what he can about the kidnapper, all while trying to locate and save his new love.