Viscount Olin Vogel, a personal emissary of Archon Katrina Steiner, has prepared a pact with the nearby Oberon Confederation by which they will garrison the world, freeing up Carlyle's Commandos for duty elsewhere.
Briefed on the situation and urged to leave the house because helping him means trouble, Grayson decides to seek out his girlfriend Mara who happens to be the daughter of Defense Minister Stannic.
He escapes and meets another off-world fugitive who reveals himself as Renfred Tor, an independent starship captain who had been hired to work for House Mailai by one of their principal traders, Proctor Sinvalie.
Tor decides to head for the starport and hire out as a tech, while Grayson retreats to Thunder Rift outside the city with a stolen militia hovercraft skimmer to think the situation over.
By a combination of battlefield instincts and pure chance, he ends up commandeering a militia skimmer with a heavy machine gun and uses this to kill the pilot of a Wasp in battle, thereby rallying the defenders and leading them to victory over the attacking pirates.
He even manages to corner an overheating Locust in a dead end and forces the pilot (Lori Kalmar) to surrender by means of a shoulder-mounted inferno launcher, thereby capturing the damaged but functional 'Mech.
Over his objections he is then tasked with creating a specialized anti-'Mech combat formation, the Trellwan Lancers, from the two captured 'Mechs and assorted militia and Guards forces, to fight the pirates still holding the garrison castle.
Competition and bickering between Guard and militia units makes training them difficult, and they also lack the spare parts and tech skills to repair the Wasp.
Captive Locust pilot Lori Kalmar reveals that three of the attackers including herself are inexperienced conscripts from the Oberon Confederation, forced to work for the pirates on Trellwan because of circumstances beyond their control.
A commando raid is staged against the garrison castle with orders to fatally sabotage the Shadow Hawk undergoing repairs there, but inside the hangar the 'Mech turns out to be fully functional.
Gambling on his crew still being in control of the vessel, Tor leads a commando team into the DropShip while Grayson sneaks into the garrison castle to forge flight orders in the computer.
"[2]: 70 Stephan Wieck reviewed Decision at Thunder Rift in White Wolf #7 (1987), rating it a 7 out of 10 and stated that "I highly recommend reading this book before purchasing the games.