Complex sentence: Nimoy also was interested in the series, which he saw was an intelligent mixture of science and science fiction, and continued to guest star through the show's fourth season, even after his stated 2012 retirement from acting.
Split into simpler sentences: Nimoy also was interested in the series, which he saw was an intelligent mixture of science and science fiction. Nimoy would continue to guest star through the show's fourth season, even after his stated 2012 retirement from acting.