Lionsgate has provided us with the official synopsis for director Darren Lynn Bousman's horror sequel, Saw IV, arriving in theatres on October 26:

Jigsaw and his apprentice Amanda are dead.
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Now, upon the news of Detective Kerry's murder, two seasoned FBI profilers, Agent Strahm and Agent Perez, arrive in the terrified community to assist the veteran Detective Hoffman in sifting through Jigsaw's latest grizzly remains and piecing together the puzzle.

However, when SWAT Commander Rigg is abducted and thrust into a game, the last officer untouched by Jigsaw has but ninety minutes to overcome a series of demented traps and save an old friend or face the deadly consequences.

In addition, the studio has released the main cast and the characters they play in the film:

Tobin Bell : Jigsaw

Scott Patterson : Agent Straum

Betsy Russell : Jill

Costas Mandylor : Hoffman

Lyriq Bent : Rigg

Athena Karkanis : Agent Perez

Justin Louis : Art

Simon Reynolds : Lamanna

Mike Realba : Fisk

Marty Adams : Ivan