Complex sentence: In "Dark Passage", Dr. Gabriel Rylander had injected the Omega-1 Nanite into Rex: that nanite later saved Rex's life from drowning in "The Swarm", when it restarted his heart.
Split into simpler sentences: In "Dark Passage", Dr. Gabriel Rylander had injected the Omega-1 Nanite into Rex. That nanite later saved Rex's life in "The Swarm" when Rex drowned and it restarted his heart.