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.

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.

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