People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint - it's more like a big ball of wibbly wobbly... time-y wimey... stuff.
The TARDIS, in her infinite wisdom, has connected all of her favourite people to her own network.
This network stretches out through time and space and, whether by accident or design, connects to multiple points in time, so that not just one Jack Harkness is online but eight!
Imagine the possibilities...
Wanted: Jack Harkness, Ianto Jones, Owen Harper, Toshiko Sato, Gwen Cooper, Suzie Costello, Rhys Williams, Andy Davidson, Rory Williams, Rose Tyler, Donna Noble, Martha Jones, Mickey Smith, The Doctor [9], Lucy Saxon, John Hart, Time Agents, Torchwood agents from the 70's, and more!