Colonel Michael Donovan, a soldier of fortune and former U.S. Marine captain, is fed up with his latest job: keeping spoiled playboy George Foster out of trouble from women and liquor in Paris.

Thus, he is eager to accept when revolutionaries Valdis and Ledgard want to hire him to kidnap King Peter II, ruler of a country somewhere in the Balkans.

With Foster's help, Donovan defeats the enemy by hand holding a M1917 Browning machine gun where he kills or captures all 250 of Gino's men just in time to save Peter from a firing squad.

Although there are long moments when nothing happens, the picture is rousingly comic in the scenes devoted to Colonel Mike's battles, and it has some claim to fame in presenting the most amazing co-starring team in screen history – Victor McLaglen and little Freddie Bartholomew.

"[2] Writing for The Spectator in 1936, Graham Greene gave the film a poor review, summarizing it as one that "should be carefully avoided", and said Freddie Bartholomew "recites his words by rote".

[3] The 2015 DVD Talk review of the film calls it "an enjoyable – if often sketchy – big screen adventure", adding that "To its credit, you don't really care that Professional Soldier bounces nonsensically around ... because the dialogue is light and amusing ... while the players, particularly McLaglen and Bartholomew, are charming together.