'Upsilon' was a British naval undertaking to land an agent on the west coast of German-occupied Norway in the area to the north of the Lofoten islands group (November 1942).

Hugo Munthe-Kaas, who had fought in the Norwegian campaign of 1940 when aged 19 and then been demobilised from the Norwegian army before escaping to the UK in April 1942, was recruited by the Special Intelligence Service. The British need for intelligence from Norway increased steadily after German major warships, most especially the battleship Tirpitz, had been transferred to that occupied country, from which they threatened the arctic convoys which carried British and US weapons, equipment and supplies to the ports of the northern USSR. Thus it became important for the British to establish a network of radio-equipped intelligence-gathering stations from northern Nordland to Troms, so that intelligence about German ship movements could be gathered and sent to the UK.

Munthe-Kaas was assigned such a mission, contributing to the creation of a network of agents with radios in July 1942, and during the night of 15 July 1942 he was delivered to Langøya in Vesterålen by a Norwegian Consolidated PBY Catalina flying boat. He established a radio station and in the summer of 1942 worked to organise the intelligence network that monitored the movements of Tirpitz in the waters of northern Norway.

At the end of this first successful operation in Norway, Munthe-Kaas travelled to neutral Sweden, and thence back to the UK. In October 1942 Munthe-Kaas was involved in 'Upsilon', the mid-November undertaking in which he and other Norwegian soldiers, together with 60 tons of equipment, were delivered to Mefjorden on the island of Senja by the Free French submarine Junon. On his return to the UK, Munthe-Kaas worked at the Norwegian military academy in London from December 1942 to a time early in 1943.

Munthe-Kaas travelled to Norway once more in March 1943, again in Junon, for 'Upsilon III', which delivered another 60 tons of supplies, and bring back to the UK two French and two Norwegian soldiers who had been left in Mefjorden during the previous raid.The four men had managed to remain hidden with the help of people in Mefjordvær.