In the mid 1970s, Kiwi charmer Terry Clark and "Aussie" Bob Trimbole form a deadly alliance that will change the face of Australian crime forever.

Anti-drugs campaigner Donald Mackay makes deadly enemies with the Griffith mafia and corrupt NSW police. Melbourne crim Ray Chuck plans Australia's most audacious heist - the Great Bookie Robbery.

A naive young Kiwi, Allison Dine, is swept off her feet by Terry Clark, becoming his lover, and most inventive drug courier.

Melbourne standover men Brian and Les Kane have ventured into unfamiliar territory by providing protection for Bob Trimbole's Melbourne heroin dealers. But things turn sour when dealer Wayne Robb is robbed of $200,000 cash on the Kane's turf. The Kane's suspect it was Vinnie Mikkelsen and Laurie Prendergast, two of Ray Chuck's mates from the Great Bookie Robbery.

Brian Kane plans Australia's most audacious gangland murder to avenge the death of his brother Les. Meanwhile, Ray Chuck forms an alliance with Mark "Chopper" Read in a Melbourne jail. Also, aspiring hitman Chris Flannery finally gets his big break and Terry's growing drug empire is under threat from problems in Australia and Singapore.

Aussie Bob Trimbole is pressured to arrange a gangland hit on his friend and colleague, Allison Dine. Meanwhile, paranoid and living in exile in the UK, Terry Clark must terminate his relationship with "Mr Asia". Also, George Freeman is determined to get revenge on his would be assassin, and aspiring hitman "Mr Rent-a-kill" gets his big break.

Australia's most dangerous hitman, Chris "Mr Rent-a-Kill" Flannery, moves to Sydney to build his deadly reputation. Terry Clark's UK murder trial reaches a sensational climax, but can Allison testify against her former lover? Meanwhile, Aussie Bob gets involved in arms smuggling for the IRA.

In the series finale of Underbelly, Chris "Mr Rent-a-Kill" Flannery becomes a victim of his own success. But who would dare order a hit on Australia's most dangerous hit man?