At the onset of the Second World War, Undine was a member of the 6th Submarine Flotilla.

From 26–29 August 1939, the flotilla deployed to its war bases at Dundee and Blyth.

[1] Undine was on her fourth war patrol in January 1940 when her sonar failed due to a leak.

Undine was at 50 feet (15 m) and proceeding blind due to the loss of sonar.

After five minutes with no further attacks, she raised her periscope and was heavily damaged by an explosion.