MR-4 touches down on Mars without incident. Looking through the periscope they see nothing but a stark landscape. There appears to be no life for miles. Harcourt Brown (George Coulouris) believes that they landed in a Sahara-like desert and that they will be welcomed shortly. Conway Henderson (Gerald Flood) is more realistic.
According to Henderson they only have fifteen hours before they lose their window to safely return to Earth. With only two days' worth of water on hand and a return trip of at least six weeks they need to find water now.
They unload a small skiff and load it with water canisters. Henderson, Mary Meadows (Pamela Barney) and Brown head for the northern polar cap in search of water. Geoff (Stewart Guidotti) and Margaret (Hester Cameron) stay with the ship and monitor the radio. While they are headed toward the polar cap a dust storm approaches. During the storm Brown separates himself from Henderson and Mary. He leaves the water canister and takes off with the skiff. Henderson and Mary believe he has fallen into a crevasse, so they continue without him.
When they reach the cap, they fill their canisters with snow. As they begin to head back a Martian rainstorm comes up. The rain triggers the planets life form, a lichen-like substance, to rapidly grow. The lichen begins to try to overtake Henderson and Mary.
Brown returns to the ship and convinces Geoff and Margaret to head south with him. They climb up a ridge where Brown expects to see a Martian city on the other side. What he sees is not what he expects.
During the sixties, lower budget shows were often recorded on videotape. The tapes were then wiped and reused. This is what happened to the Pathfinder series, so at one point they were believed to be lost. Recordings of the series were made for sales to overseas broadcasters. At some point various episodes of the Pathfinder series recordings were returned to ITV from other overseas broadcasters. In 2011 the last missing episode was discovered in ITV’s own archives.