Ikarie XB 1 is a spaceship en route to Alpha Centauri, its mission to discover extraterrestrial life on the mysterious white planet. The crew of around 40 people must deal with the dangers and existential consequences of travelling through deep space, light-years away from Earth. The dangers include an ancient 20th Century (the film is set in the 22nd Century) derelict spacecraft which carries nuclear weapons, and a dark star which emits radiation powerful enough to impair the crew members' normal biological functions, threatening to send them into a sleep from which they will never wake. The existential aspects include the knowledge of a time dilation phenomenon meaning that when they return to Earth, everyone there will have aged 15 years more than they have themselves, as well as the extreme boredom inherent in their long journey.
It is an austere, understated film, in which even the most dramatic situations are not sensationalised. It reflects the optimism of communism which still existed behind the Iron Curtain at that time (1963) - aside from the occasional minor conflicts which are unavoidable in their isolated, claustrophobic situation, the crew is presented as a harmonious group working together, and their mission is one of peace.
It's a fascinating curiosity and it is thanks to the excellent Second Run DVD company that this film (and a great many others) has been saved from obscurity and can be seen is this lovingly restored state. There are some nice extras on the Blu-ray as well: an illuminating discussion of the film by Kim Newman; an interesting short film about mathematics from the same era as the film; and, best of all, alternate opening and closing scenes from the dubbed American version, complete with a tagged-on Planet of the Apes ending.