Overture: A Captain Jack Adventure (comic story) has been suggested by OncomingStorm12th.
Talk about it here.
Overture was the tenth and final story printed in Torchwood The Official Magazine.
As a group of hostile aliens break into the Torchwood Three hub, the Commodore hands Jack Harkness a box. Opening it, he gets sent into the future, and is given instructions on just how to handle the aliens.
Jack's sitting in a club on 7 August 1941, watching around for possible aliens, when the band uncover their true appearance, and suck all life out of the singer. As they then proceed to attack everyone in sight, Jack escapes and runs to the Hub where he finds, much to his annoyance, that everyone had already left — nearly everyone. As the aliens break through the last layer of security and enter the main room of the Hub, the Commodore hands Jack a safe.
Opening it, he watches over events of either his past or future. On New Year's Eve of soon-to-be 2067, in a bar on the planet Zog, the other Jack is given instructions: he must get his 1941 self to sing a song, which will effectively "switch off" the aliens. He is given a rift-safe thought safe, which is what had brought Jack back to witness this.
- The Commodore is surprised that Jack can sing, calling his singing "pitch perfect". In fact, John Barrowman, the actor who plays Jack on TV, is also a professional singer and has released several albums.
- Jack mentions going through World War II a second time, (TV: The Empty Child, Utopia) and states that he couldn't go anywhere near his previous self's life due to the temporal paradoxes it would cause. (TV: Exit Wounds, The Three Doctors, Mawdryn Undead, Father's Day)
- He also mentions his memory being wiped. (TV: The Empty Child)
- Jack complains about the Hub's security, saying he'd like "a reinforced steel and concrete rolling door" — as he would later have installed by TV: Everything Changes. Unfortunately, it would be breached in TV: The Stolen Earth by a Dalek.