Seems like this to me:
Fallout 1: PC is sent from the Vault to find a water chip but gets caught up in the super mutant deal and must kill the mutant's leader, the Master.
Fallout 2: PC goes looking for a G.E.C.K. and eventually destroys the Enclave.
Fallout 3: PC is kicked out of the Vault and goes looking for his/her father but gets caught up in nullifying the super mutant threat and finding a G.E.C.K. in order to supply water to the masses and save the wasteland from the Enclave. (Wow it really does sound like the plot of 1 and 2 mashed together)
Spoiler Fallout: New Vegas: PC is ambushed and has his/her package stolen before getting shot in the face and left for dead. The ensuing game revolves around finding Benny in order to recover the package and finish the delivery. However after finding Benny and extracting the information needed, the courier discovers Benny's plot to wrest control of New Vegas from Mr. House and is forced to decide who, if anyone, will end up in control of the region.
Fallout 3 appeared to be the most unoriginal story. However, there are similarities in regards to the basic plot layout like how the PC goes from zero to hero, the struggle of good versus evil (which New Vegas seems to avoid), etc.