When Earth's "Pathfinder Project" transmits its first block of data to Voyager, the Doctor receives disturbing news — Lewis Zimmerman, the creator of modern holography and the Doctor's program, is dying.
Hoping to save this "father" he never met, whose likeness he shares, the Doctor's program is transmitted back to the Alpha Quadrant.
Instead of being pleased with the Doctor's attempts to help, Doc Zimmerman is completely standoffish. Zimmerman tells the Doctor that he is simply a Mark One hologram. He has been examined by the Mark Two, Mark Three and Mark Four, in addition to the finest "real" doctors in Starfleet, and none of them have been able to help him. Barclay points out that the Doctor has been running almost continuously for six years and that he has seen things that most doctors couldn't even imagine. Zimmerman eventually becomes frustrated with all of the Doctor's questioning and he abruptly instructs the computer to transmit the Doctor to the living quarters.
Meanwhile, Barclay pleads with Counselor Troi to return from the U.S.S. Enterprise in order to counsel Doc Zimmerman. However, when she does show up, things seem to be worse between the two doctors. Troi tries to convince Zimmerman that the Doctor has developed a promising treatment.
While the arguing continues between the Doctor and Zimmerman, Barclay discovers a problem with the Doctor's program. His program is unraveling and there is nothing that Barclay can do because the damage is too severe. Zimmerman is the only one who can save the Doctor, but of course he refuses to help a "replaceable" computer program.
In an attempt to convince him to help the Doctor, Zimmerman's right-hand hologram Haley tells him that she will ask to be installed elsewhere unless he does something to save the Doctor.
Zimmerman successfully identifies an error within the Doctor's pattern buffer. Thankfully, the Doctor is able to stop him from changing his personality subroutine as well. In the process, the two seem to make peace with each other and Doc Zimmerman finally agrees to treatment.