Before Harcourt Brown (George Coulouris) can launch MR-4 from the surface of Mars, the rocket becomes infested with lichen.  Communication with Conway Henderson (Gerald Flood) gives Brown the information he needs to stop the lichen by freezing it.  The cold also incapacitates Brown giving Henderson and the others time to open the hatch and re-take the ship.

Mary Meadows (Pamela Barney) extracts water from the frozen lichen allowing the crew to replace their water supplies.  The crew of MR-4 is far from being out of trouble.  The delay in getting access to the ship has resulted in the orbits of Earth and Mars pulling away from each other.  Even if they launch now, they will not be able to catch up to Earth.

Brown had developed a theory that they could use the gravitational pull of the Sun to slingshot themselves around the Sun toward Earth.  It is only a theory, and very risky, but there are no other options available to them.  They have no choice but to try it.  As they get closer to the Sun they begin to succumb to its radiation.  Just as they are about to all pass out, an eclipse of the Sun and Mercury blocks the Sun’s radiation.  By now they have past the deflection point and are being drawn by Mercury’s gravitational pull.

In a last-ditch effort to save themselves, the crew of MR-4 look to Earth for help.  Buchan Island has made many satellite fixes on Mercury.  Using the ship’s autopilot, they release control of the vessel to Buchan Island’s computers in hopes that they can pilot the rocket back to Earth.      

The sling shot method of acceleration has been used as a plot point in Star Trek at least twice.  The first time was in season one episode nineteen of the original series, “Tomorrow is Today”.  The second time was in the Star Trek film “Star Trek IV: The Voyage Home”.  Both were done to travel back in time.  The sling shot method, or gravitational assist, is a tried-and-true trope in a plethora of science fiction stories but there are practical applications of the method that have actually been used by scientists.  Voyager 1, for example, used gravity assist while passing both Jupiter and Saturn to increase its speed leaving the solar system.