More astronaut-cosplaying aliens, palpably inappropriate music, and foreign conspirators from foreign countries
Three astronauts have been out of contact for 7 months, and the rescue astronaut sent after them also goes incommunicado. Bingo. In step U.N.I.T. and THE DOCTOR, who quickly dispatch the latter into space to rescue the chap who was sent to rescue the chaps.
Meanwhile, three astronauts do return to earth. But are they perhaps some dastardly, Martian invasion force? Or are they radioactive space ambassadors used for nefarious purposes by a renegade general, hell-bent on interstellar, thermonuclear war?