Dr. Slate, Big Guy and Rusty find out that the Legion Ex Machina's mysterious ""final assembly"" is taking place in a network of underground tunnels. When Big Guy and Rusty find the tunnels, they derail a robotic train. Inside the massive train's cargo hold is a gigantic hand that boasts a built-in high-tech cannon. With their plan uncovered, the Legion Ex Machina members decide to activate their latest creation ahead of schedule and missing a hand.