At the end of the last episode you could here the 'Tardis' when Cpt.Jack disappeared.So my thoughts are he is a'Time Lord' like Dr.Who or the Doctor came to give him a lift somewhere
.If it is that he is a 'Time Lord' it may be time for a switch in actors playing the part which is usually what happens in the Dr.Who series.Tinks wont be very happy if that happens.