Smash and Grab, two antiquated robots who live on a futuristic Mars-like planet, have spent all of their lives trapped in a giant train fueled by glowing rocks. Smash breaks the rocks apart, and tosses them to Grab, who feeds them to the engine. They attempt to amuse themselves but are hindered from moving freely about the room or seeing out the window by their power cables, which are also kept short enough that they cannot touch each other, even to high-five after a good day's work, much to Smash's frustration.
One day, Smash is able to build a stack of rocks and see out the window. It learns there is a world outside the train, and that all the free robots move about unhindered thanks to spherical batteries that attach via cable to their power ports. Seeing that some of these batteries have been loaded onto the train, Smash cuts its power cable and uses what little power it has left to steal two batteries, attaching one to itself and the other to Grab so they can flee the engine room together.
Security robots pursue Smash and Grab and attempt to kill them before they reach freedom. Trapped atop the train, Smash catches and throws security robots to Grab when they come through the door. Grab flings them against obstacles the train passes, noticing their batteries explode if damaged. One security robot manages to shoot Grab's arm off, and Grab becomes temporarily deactivated. Smash attempts to revive its friend and is cornered by eight security robots. Angered, Smash deliberately damages its battery and throws it at the security robots. The battery explodes, causing a chain reaction which kills the security robots and derails the train.
In the crater left by the explosion, Grab reactivates, finds a powered down but intact Smash, and connects its battery to Smash via a cable so they can share its power. The two high five for the first time, and set out to explore the outside world together.