There are many rumored stories oh Jab Tak Hai Jaan floating around on net.. so after watching all the song trailers of the film this is what i feel can be the story.. ( just my assumptions )
srk is in London as a student to study music.. meets katrina, they become frnds, Srk fells in love.. the romance continues.. either his parents call him back to india so that he can serve his nation ( his father Rishi Kapoor has this wish) or SRK- Katrina Kaif fell apart due to some mis understandings.. katrina refuses to go to india with SRK.. he comes and joins army.. Anushka meets him as a reporter.. falls for him.. while he is still nursing his broken heart which makes him stern.. she tries to find out why is he like this and his past..
either srk tells his past as a flashback or she gets to know through his diary or something.. @anushka decides that the couple should meet again.. tries and takes srk back to london with her..
now in the climax either they meet or some twists where he may chose anushka over katrina..! but that can be the basic plot as per my assumptions.. just hoping the story has ample amount of twists :)