This show aired out of order, since it is Part 2 of a story that began in We're Not Coming Back (1965) which aired many weeks later. It is easy to tell due to characters discussing the shuttle mission and referring to actions that occurred in Yugoslavia.
Jack Lord's character Preston Gallagher divulges that his childhood nickname for Joe Gallagher was "Danzo" -- an interesting precursor to "Danno" on Hawaii 5-0!