Major Stringer (José Ferrer) is put in charge of a group of volunteers much to the annoyance of veteran Captain Thompson (Trevor Howard) who does not agree with Stringer's methods leading to them clashing over the training of these men. But then these men have volunteered for what many consider to be a suicide mission as the plan is to use kayaks to covertly enter a German controlled shipyard in France and destroy the whole thing.
Whilst "The Cockleshell Heroes" is based upon Operation Frankton it is more of a movie which follows a traditional set up. We get a first half which sees some misfits training and a second half which sees these men on a dangerous war time mission. That doesn't make it a weak movie but it does mean "The Cockleshell Heroes" is incredibly familiar as we get a first half which is filled with the humour of these men training. Scenes of these men running in their swimming trunks to dispose of a bomb at sea to them winding up a training officer is all entertaining but nothing out of the ordinary.
After that entertaining first half we then get the more drama/action focused second half as we have the actual mission and it works well because going from fun to drama is always a nice way to go with war movies. As to how authentic this second half is I can't say but like with the first half the second half follows a familiar style you will come across in many a war movie which means whilst entertaining it doesn't break the mould.
In truth whilst I find "The Cockleshell Heroes" an entertaining war movie what makes it more entertaining is the behind the scenes stories. Evidently Bryan Forbes wrote the script but director José Ferrer had it rewritten by Richard Maibaum. After producer Irving Allen realised that Ferrer's vision of the movie was not only lacking in humour but ignored many of the supporting cast and characters in favour of Ferrer he had Forbes rewrite some of Maibaum's script and secretly direct some new sequences. This lead to Ferrer reportedly walking away from the movie. And when you learn that you can sort of see that being the case with some clever editing when it comes to those humorous scenes during the training sequences.
What this all boils down to is that "The Cockleshell Heroes" is certainly an entertaining war movie but despite its operation specific storyline it runs with a familiar set up of a humorous first half featuring training sessions and then the excitement of a second half mission with both action and danger taking precedence over the humour.