Having plucked unwitting crews from different sources all over time and space,the Eternals are finding many of the ships in the race are being mysteriously destroyed. While investigating outside with the doctor and Tegan Turlough,no longer able to cope with the pressure put upon him by the Black Guardian goes overboard and is captured by the ambitious Captain Wrack. He is able to deduce out that Wrack has been tapping into the power of the Black Guardian Turlough has been resisting. While attending a part of Eternals on her vessel Wrack uses Tegan as an unwitting host to an amplifier she was attempting to use to destroy all the other ships in the race. The doctor discovers this,destroys the amplifier and in the end,there is a meeting between the TARDIS crew and the two Guardians. The Black Guardian offers Turlough a choice-the key to enlightenment or the doctors life to save his own. Turlough chooses the doctor,with the key to enlightenment seeming to incinerate the Black Guardian. For his part the White Guardian leave the doctor with the sobering message that as long as the light exists,so will darkness.
This is one of the most thematically ambitious stories of the entire classic Doctor Who series. For it's low budget effects,the footage of the space ferring sailing yachts are very visually stunning. But it's also an excellent story for Turlough to learn a valuable lesson. While not inherently evil,the Eternals are very easily turned morally because they rely on the emotions of whom they call "ephemerals" such as human beings in order to survive. This is seen most evidently in Mariner's inability to comprehend his own love for Tegan. For Turlough's part his discovery that the eternal Wrack is only wishing to seek any sensation wherever it comes brings his own greed and avarice into focus. And in the end he decides to continue his travels with the doctor and put his entire affiliation with the Black Guardian behind him. The core of this story though is the concept that for anyone wishing for eternal existence that it's important to have an understanding of our emotions and sensations before giving them up for a self centered concern.