Aircraft are being shot down by a large black plane with a big "X" painted on the wing. The chief suspects are invited for the weekend to an old dark mansion.

A boxer is framed for murder after an opponent dies in the ring.

Betty Bryant is an ambitious newspaper reporter in love with Dan Barton, a member of a big-city Emergency Squad who are trained to deal with riots, cave-in, explosions, fires and other emergencies where lives are at stake. Slade Wiley, an unscrupulous tunnel builder, finds that his low bid on the Newford Tunnel project is causing him to lose a lot of money, and has underworld leader Nick Burton set off blasts to frighten the stockholders into selling their shares at a low price so he can buy up the stock. Betty is investigating the deal when Wiley and Burton take her on a "tour trip" to the tunnel.