When Mary Ann and Pete Ventrino are late returning from Gary and Molly Burton's barbecue party, their child-minder Andria Carrasco goes to find them - and uncovers a harrowing scene.

All four have been bound, gagged and poisoned to death in Gary's garage in a meticulously planned attack. The killers left no clues except for a threatening note personally addressed to Sam, which suggests the motive may be linked to the mixed marriage of one of the deceased couples. Against the advice of his colleagues, Sam is determined to solve the case, but is he putting his own life, as well as the lives of those he loves most, in grave danger?