Complex sentence: Keiji is killed on his first sortie, but through some inexplicable phenomenon wakes up having returned to the day before the battle, only to find himself caught in a time loop as his death and resurrection repeats time and time again.
Split into simpler sentences: Keiji is killed on his first sortie, but through some inexplicable phenomenon wakes up having returned to the day before the battle. This retroversal continues and he now finds himself caught in a time loop as his death and resurrection repeats time and time again.