In early 1939 the Polish team supervising the building of the ship noticed a significant slowdown in her construction, which it attributed to the action of German agents.

On 18 April Sęp arrived in Gdynia, entering the harbour on her electric engines, and was officially commissioned into the Polish navy.

[5] There is a common version, that a magnetic primer failed, and the torpedo blew up before the target, damaging it, however Polish documents prove, that an explosion was detected well behind the stern, and made no harm.

[5] German submarines did not want to risk a surface combat with faster and better armed Polish ships, hence they undertook hasty actions.

[5] With her position clearly revealed to the enemy, the submarine left her assigned sector and began to sail in the direction of Gotland Island.

On 13 September the submarine received orders permitting her to sail to England if possible, and otherwise to be interned in a neutral Swedish port.

In early 1940 attempts were made for Sęp and the two other interned Polish submarines to be released by Sweden and allowed to proceed to Britain, after necessary repairs.

The Polish submarines were moved around various Swedish ports in the course of the war, and even received maintenance work, but were not allowed to leave.

After the war ended, on 23 June 1945 a Polish Military Commission arrived in Sweden to arrange for the return of the interned submarines to Poland.