Captain Jack Harkness was an American serving as an RAF Group Captain, stationed in Cardiff with the famous 133 Squadron. He had a brief, intense, affair with the ex-Time Agent "Captain Jack Harkness", a man who had appropriated his name.
While based in the World War II time period, (TV: The Empty Child, The Doctor Dances) Captain Jack Harkness took the name and identity of an American airman he knew would die in battle on 21 January 1941. Jack hadn't known anything about the other Jack as a person, or even what he looked like until, transported from the 21st century to 1941 with Toshiko Sato, they met each other. In 1941 again, the future Jack adopted the name Captain James Harper.
On the night of 20 January 1941, Jack and "Harper" met. The two men felt an immediate attraction. Both had experience of war and of close friends having died in it. Jack had to overcome the fear and reluctance over moving the relationship to a sexual level. Jack also demonstrated a strong compassion for others, backing up "Harper"'s story about Toshiko Sato working as a code-breaker for the government when she nearly fell victim to anti-Japanese prejudice. For his part, "James" knew Jack would die the next day.
Jack eventually struck up the courage to dance with "James" in front of a party of servicemen and their guests. As the Cardiff rift transported "James" and Toshiko back to their own time, the two men kissed passionately, leaving both in tears.
The next day, 21 January 1941, during a training exercise Jack was taking part in, German aircraft appeared. Captain Jack Harkness died in the surprise attack, but managed to save the lives of all of the men under his command.