In the stormy seas of Galway Bay, Jim Prendergast and his treasured yacht Larinita are doing their best to compete in a local race. Along with him are his wife and daughter. As the waves swell more and more and a fog settles in, Jim notices a freighter not far off heading their way and closing in fast. He tries to avoid them, but can't. The larger ship rams into the smaller boat and the yacht quickly sinks in the cold Atlantic water. Jim survives but his wife and daughter drown. Thus begins his personal quest for revenge which will lead him on a dark journey involving IRA gun-running and an international terrorist plot of major proportions aimed at destroying the peace talks once and for all. Prendergast's tragedy comes to the attention of Jonathan Cooke, a shadowy figure in the British Embassy in Dublin, who suspects that the freighter was transporting guns for a maverick IRA unit which has a female German traveller with them. Manipulated by the MI6, Prendergast is dragged unwittingly into refurbishing and then captaining a supply ship for the IRA to the Mediterranean to pick up some "special" cargo. The plot grows more frightening when Prendergast learns about possible Libyan involvement and a ruthless U.S. mercenary helicopter pilot joins the crew and has a strange structure built on top of the ship. Undertow reaches its climax in a violent and bloody confrontation back where it had all begun, on the stormy seas of Galway Bay.