It's 1971: Elmo McElroy (Samuel L. Jackson), fresh from graduating college with a degree in pharmacology, is pulled over by a patrol car and caught smoking marijuana. The police officer arrests him, thus preventing him from getting a legitimate job. Fast forward to the present day: a drug lord called the Lizard (Meat Loaf) calls a meeting with his organization, hoping to sell a brand new substance invented by McElroy. The deal goes awry when McElroy, in a bid to escape from the Lizard's control, blows up the building. Most inside are killed, but the Lizard manages to escape. Revenge very much on his mind, he contacts Dakota (Emily Mortimer), a contract killer, to take care of McElroy. Although Dakota initially refuses, it emerges that she is in a great deal of debt to the Lizard, and he offers to `clear her slate` and give her a $250,000 bonus to complete the hit on McElroy.
McElroy heads for England in the hope of selling his product there. He arranges a meeting with Leopold Durant (Ricky Tomlinson), head of a local criminal organization, and Felix DeSouza (Robert Carlyle), an ex-hitman, is sent to escort McElroy in exchange for football tickets. At the meeting, McElroy makes his pitch: ecstacy uses serotonin, opiates use dopamine (produced by the brain during sex), amphetamines use adrenaline, cocaine stimulates synaptic activity. He claims his product, POS 51, a synthetic that can be produced with minimal facilities, is precisely 51 times as potent as any of those. A second opinion from a local chemist confirms this and Durant is impressed.
However, while the deal is taking place, Dakota is watching through a rifle scope waiting for her chance to kill McElroy. But before she can pull the trigger, the Lizard calls, now with greed his main priority. He wants the formula for McElroy's drug. Everyone else at the meeting is fair game. Making their escape, DeSouza is shot in the rear on purpose; as it turns out, Dakota and DeSouza have a history. As they leave the hotel, the two are attacked by a bunch of tattooed skinheads, who have somehow found out about the drug. McElroy, however, makes short work of them armed with nothing but his golf clubs.