Complex sentence: She reveals that one of the ships of an incoming convoy has been secretly replaced by another filled with explosives, which is to be detonated when they reach Halifax, wrecking the vital port, a plan inspired by a devastating accident of the First World War.
Split into simpler sentences: She reveals that one of the ships of an incoming convoy has been secretly replaced by another filled with explosives, which is to be detonated when the convoy reaches Halifax. It is planned that the resulting explosion will wreck the vital port, a plan inspired by the devastating accident of the First World War.