Mars in its infancy looked a lot like early Earth, with a thicker atmosphere and liquid water on its surface, and this film tracks how planetary scientists use that resemblance to test theories about our own planet's development. Researchers walk through orbiter and rover data, from dried river channels to mineral deposits that only form in the presence of water, building the case for a warmer, wetter Martian past before something stripped its atmosphere away. The film covers the spacecraft and instruments built to hunt for those signatures, and the harder question underneath the mission plans: whether conditions billion of years ago were stable enough for microbial life to take hold before Mars turned hostile. Interviews with scientists frame the search less as a hunt for present-day aliens and more as a comparative planetology problem, using Mars as a control case for how habitability can be gained and lost. It closes on what current and planned missions are still looking for.