Earth is attacked by alien spacecraft from Venus but a Japanese scientist has constructed a space craft, the Gohten, with which he may save humanity. That's the plot for Wakusei Daisenso or Great Planet War: THE WAR IN SPACE as it is know in Japan.
Round our way we know it simply as The War in Space, that film where the spacesuits look a lot like those seen in 2001... and just because they can.