Mutt and Jeff were two Norwegian spies who worked for the United Kingdom and MI5 and were members of the Double Cross System.
In April 1941 two Norwegians, John "Helge" Moe (Mutt) and Tor Glad (Jeff) fetched up on a remote Aberdeenshire beach in Scotland, having travelled by seaplane and rubber dinghy.
[1] Mutt and Jeff's mission was supposed to be one of sabotage, as well as having a secondary intelligence role, reporting (via wireless) military locations, deployments and civilian morale.
MI5 used Mutt and Jeff's radio sets to relay false information, leading the Germans to believe that the United Kingdom intended to invade Norway (this activity was a component of Operation Fortitude).
To ensure the German handlers did not suspect Mutt and Jeff of having sold out, MI5 arranged for fake sabotage operations - supposedly the work of the pair - to take place, in one case going so far as to set off a controlled explosion at a power plant which was widely reported in the local media as a genuine attack.