The Doctor is attempting to set up the TARDIS to defend against two Sontaran troopers that are on board, but inadventently activates a matter transporter bringing first Tegan and then a young boy named Gareth Jenkins to the console room - the latter dressed identically to the Doctor! The boy assists the Doctor in operating the TARDIS in readiness, and is revealed by the Sontarans as they arrive as a key defender of Earth in its future, thwarting an invasion - he then saves the day by throwing a switch that releases a gas to kill the Sontarans.