Dan, Steve, Betty and Fitzhugh witness a wounded man stumble into an apartment. They try to call for help, but the man tells them to call "Dr. North". They do so, and get a vague message telling them to wait. They hide as a female doctor comes for the man, Kamber, who is concerned about the wiring job he did and wants to know where a key is. Kamber claims he lost it, so the doctor lets him die after mentioning her plan to blow up the city!
Spotting the little people, North leaves a bomb which they manage to avoid, although Fitzhugh's leg is broken. The group can't move Fitzhugh, and almost kill him with the injection of an unknown "wide spectrum antibiotic" that puts him in a brief coma.
While the others leave to investigate, Betty stays with Fitzhugh, and they are both captured by Kobick, who is tracking Kamber. Kobick doesn't believe their story about the bombs. The men find the coin and the concealed map overlay. Steve tries to convince Kobick of their story, and the Inspector reluctantly checks it out.
Dan and Mark manage to track Dr. North, and they and Steve get captured by her. Steve injects her with the antibiotic they used on Fitzhugh, then sets the clock ahead. When she awakens, "North" doesn't believe them, but the Earthlings rig a fake call to her travel office (actually Valerie on another phone), and she tells them where the bombs are.
While Kobick and his men defuse the bombs, Steve breaks back into SID HQ and frees Betty and Fitzhugh. Kobick corners him but Steve buys his freedom with the location of "North", and Kobick gets the credit for breaking the case.