Arc 15.5:  The Killing Ground

Written by Jim Holdaway (artist), Peter O'Donnell (writer)

Strips: A1 - A36

Dates: 10/07/1968 - 11/16/1968

An old enemy, dying quickly of cancer, captures Modesty and Willie and hires three killers to hunt them down for his final amusement. According to Wikipedia: this story was "produced to Scottish newspapers after an industrial dispute in England". It is definitely canon but outside the normal flow of the stories.