Complex sentence: When he is late getting back for a patrol, Jim's right hand man, "Hap" Smith (Paul Kelly), secretly takes his place, despite being grounded by Jim because his vision had deteriorated, particularly at night.
Split into simpler sentences: When he is late getting back for a patrol, Jim's right hand man, "Hap" Smith (Paul Kelly), secretly takes his place. Unfortunately, he had been grounded because his vision had deteriorated, particularly at night.