Who Or How

Season 1 Episode 3 The Doctor arrives at the place he was traveling to, but realizes that he locked The TARDIS... with the key inside. He then tries to use his sonic screwdriver, but that doesn't work. Then, the Master appears and tries kill the Doctor. The Doctor then used the sonic screwdriver to destroy the Master, and he malfunctions. The Doctor finally unlocks the TARDIS and goes in, just in time before the Robot exploded...