[1] Thistle, under the command of Lt. Wilfrid Frederick Haselfoot, was ordered to patrol off Stavanger, and to sink any enemy vessel that she might spot in the harbour, since British authorities believed that a German invasion of Norway was imminent.

On 10 April, Thistle signaled her intention to comply with this order and that she had two torpedoes remaining after an unsuccessful attack on a U-boat.

It was later discovered that U-4, the U-boat Thistle had previously attacked, had sighted the submarine on the surface and sunk her with torpedoes.

U-4 observed one torpedo passing ten meters ahead and evaded further underwater attacks by crash diving.

The wreck of the Thistle, missing its tower,[3] was discovered by the Norwegian Institute of Marine Research in the spring of 2023, at 160 meters of depth.