When David looked up and saw Meg halfway across the swamp sprinting away from the Doctor, he stopped helping Ace fix the generator and started running over to protect her. He'd teach that laughing freak to pick on someone his own size. Hefting his flashlight by the leather grip, David picked up the pace until he caught up with them a moment later.
The Doctor had followed Meg onto some kind of house boat and was now chasing her around what appeared to be the captain's quarters. Her screams of terror pierced the eerie stillness of the swamp as he treated her with a touch of Carter's Spark. Then, as the slightly crazed Meg ran down a set of stairs past David, who was making his way up, the latter's golden opportunity presented itself.
The Doctor was busy breaking a pallet that was blocking his path when David got to the top of the stairs and pointed the flashlight. The Doctor took a swing, but had one arm out in front of his eyes to shield from the odd bulb's blinding rays, and the bare knuckle brawler easily ducked out of the way.
Instead of flying back down the stairs and as far away from the boat as quickly as possible, David turned a corner, found himself inside the captain's quarters and ducked low. Right as he did this, the generator he was working on with Ace lit up a wide section of the swamp. Practically everyone could see Ace's baseball-capped silhouette fleeing the scene, and the Doctor wasted no time going down there to catch him. Ace was in trouble. Both eager to keep everyone under his protection safe and secretly itching for a piece of action, David readily followed behind.