A/N: I don't own Halo, but I own the interpretive head canon. That's about it. Everything else is from the dialogue and cutscenes in Halo 4. Hope you enjoy!
John's hand was burning, but he kept holding on. Five fingers clenched the precipice of the light bridge with enough force to crack his bones. With only one hand weakly holding him, The Master Chief's body swung out to the side, as his eyes were full of the vision of some dark swirling matter that loomed below him. "So much has been sacrificed, and I absolutely will not let her down..." he spoke inwardly, prodding his muscles to keep moving.
John paused for a moment, confused with his own thoughts. "I won't let humanity down," he revised. He swung his other arm toward the edge of the bridge and the momentum allowed his hand to capture the rounded surface. His muscles were hardly working, but he pushed on. Forcing his body to inch toward the top surface of the light bridge, he caught sight of the Didact, shackled down by Cortana's multiple rampant holograms.
The nuke that desperately needed to be detonated was far out the Master Chief's reach, so he was forced to improvise. But he seemed to be quite good at that. The cogs in his brain began to work, and before he could consciously send any commands to his muscles to carry out an order, his body had flung itself onto the surface and had begun racing forward. He felt a Promethean grenade clutched in his hand that he didn't remember retrieving. As the Master Chief shoved the object at the figure on the floor, he was suddenly overcome.
It hadn't worked.
He must've missed somehow… John was suddenly yanked into the air by the forerunner's immense telepathic power. He struggled, but of course it was inane. But he continued to fight, like he always would. Nothing was a lost cause as long as there was one fool left to fight for it. Two fools, in this case… He thought briefly of Cortana.
A loud sound then drags the Chief brokenly back to reality as quickly as he had left it. John's eyes suddenly snapped to the forerunner's, and it registered in his mind, like a brick smashing into his brain. His grenade had made it to its destination. The explosion echoes throughout the area, and John treasures the noise, briefly forgetting about every sound he's ever heard; only recognizing the feeling of a lost hope now recovered.
The Didact's eyes lost their light and direction seemingly all at once. The forerunner clutched blankly at his mutilated chest and tumbled over the edge of the light bridge, lost to a seemingly endless oblivion. John was released from the force of power that shackled him to air, but was reminded that the mission remained still incomplete. After he watched the forerunner disappear, he threw himself in the direction of the nuke.
This ship needed to be destroyed before any more harm could come to earth. That was the mission. As long as John 117 was alive and breathing, he would complete the mission. That was how he was programmed. Two ton weights must've been strapped to his feet, because he was moving in slow motion. The Master Chief's muscles would falter, and the thought of failing would terrify him enough to keep moving. When his hands finally landed on the explosive object, he wasted no time.
Without another thought, he slammed the top of the nuke, investing all of the energy and life he had left.