World War 1. A brutal war has settled into desperate trench warfare, where victories are measured in inches gained.
In a converted farm, a cache of chemcial weapons leak into the local water supply and the toxic cocktail mutates the normally harmless leeches into bloodsucking monsters on the scent for human blood.
The leeches follow the trail of the chemicals back to source and attack the German camp and slaughtering the soldiers, leaving their drained bodies in grotesque fashion before retreating to the water.
A British squad arrive to attack the base, with the intention of destroying the weapons cache, but instead they discover the silent camp and soon uncover the corpses covered with strange marks.
As the British squad puzzle over the dead soldiers, the leeches, attracted by fresh blood, emerge and attack again.
The squad barely make it out alive.
When a shadowy figure from the Secret Service asks them to go back to collect samples they are non too keen to return. But with the war at stalemate this could be the weapon to turn the tide.
So with the help of a Nurse, who's expertise is medical leeches, and a mysterious representative of the Secret Service, they return to the site of the massacre. This time armed with flamethrowers and mason jars to collect samples of the man-eaters.
Fate intervenes and as they approach they encounter a German squad, who's mission to fond out why contact with the camp has broken down and where their compatriots are.
Their normal differences are put to one side when they are all attacked by the leeches, bigger and faster than they were before.
An uncomortable alliance is forged as they formulate a plan to draw the leeches towards the British trenches and a weapon that may present their only hope of destroying the leeches.
They struggle through the night, leeches in pursuit, with suspicion between the two factions high. Through German trenches and potential betrayal towards the British trenches and their one hope of destroying the blood-sucking freaks.