Extreme angler and underwater detective Jeremy Wade goes in search of the real creature behind the enduring legend of the sea serpent - starting with a violent attack in the English Channel, and culminating in the dark depths of the Mediterranean Sea.

Something is terrorizing the Florida Keys, attacking both visitors and locals alike - striking its victims so violently it is slicing open flesh, even smashing bones. Jeremy Wade is determined to identity the assailant, but is quickly out of his depth.