you guys did a pretty good job. >> reporter: a worldwide search launched by "20/20" located his is about to replace the mementos that he's carried close to his that handful of photos. >> yeah. it's all i had, all these years so i just, wherever i went they went, so. >> reporter: we've brought them all to new york. as the twins approach, the expectations are building to a crescendo. >> i think we're just real

in may of 1976, a plane from seoul, south korea, arrives at new york's kennedy airport. aboard are two korean children, brother and sister, twins. allen thomas' lost children coming to live in the same country as their father, if only he had known. >> reporter: here she is, the twin allen named sandra now called susan, all grown up. the little girl in all those pictures all those years ago, missing from her father's life for decades.

dad. >> come here! >> hi. >> i missed you so much. >> great to see you. >> reporter: a reunion with the most fantastic of expectations has been realized. >> you're my actual father. i'm standing right here looking at you and i still cannot get the grasp that you are my father. >> reporter: we gave the twins their first look at their original birth certificates, which allen had kept all these years. >> you see, that's the original, so. >> that was my name? james allen thomas? >> but your first name is your middle name. >> timothy james. >> reporter: and a keepsake only a parent would cherish. >> this here i used to give you