The Doctor's TARDIS materialises aboard a cargo freighter just as space pirates attack and slaughter the crew. The pirate ship is itself destroyed by an energy beam that draws the cargo freighter into a nearby asteroid belt. While trying to avoid the pirates, the Eighth Doctor and Izzy Sinclair are confronted by a Cyberman hiding in the freighter's cargo hold, and the Doctor electrocutes it; however, it survives, and is revealed to be Kroton, the only Cyberman ever to survive conversion without losing his individual identity. The TARDIS is removed from the cargo bay, and the Doctor, Izzy and Kroton investigate to find that it has been stolen by a madman named Tobal Reist, who destroyed his home planet while testing a weapon that turned out to be far more powerful than he imagined. The weapon has maintained an atmospheric bubble around this planetary fragment, enabling Reist to survive, and it has now brought him a time machine he can use to return home. The pirates kill Reist and try to seize the weapon, but Izzy grabs it and orders it to self-destruct, which it does. The asteroid city begins to break up without the weapon to hold it together, and as the surviving pirates argue over who will use the transmat unit to get to safety, the Doctor and Izzy retreat to the TARDIS and escape with Kroton.