S5·E12 The Deadliest Man Alive

The world's most deadly assassin who has left a trail of blood in history (and may even be involved in the JFK assassination), The Viper, is in town. Walker is informed and recruited by Interpol Agent Draper who's hunting the Viper now for nearly 25 years. Since the killer is in town to carry out another assassination - the target could be the visiting Israeli Ambassador - it's up to Walker to find and eliminate the Viper before he hits once again.