Richard da Costa and Alex Parkinson have set themselves a considerable challenge in this diverting documentary. The film concerns a dive that went disastrously wrong for a crew working on drilling equipment in the North Sea. An IT failure sent their vessel into a spin and left one diver, Chris Lemons, dangerously stranded at the end of straining lifeline. So far, so exciting.

The problem here is that the entire incident lasted barely half an hour and was mostly taken up with rebooting the system and trying to find manual methods for steering the submersible.

The filmmakers largely succeed. One crafty conceit may annoy documentary purists, but it’s a lot less suspect than that in the recent Oscar-nominated Minding the Gap. The divers are good talkers and the blend of dive-cam footage and dramatic recreations proves to be an effective storytelling technique.

The picture is most useful as a study of how professionals cope in extremis. We learn enough about the technology to make the story lucid, but not so much as to risk blindness by jargon. The divers react in strikingly different fashion. Most are now emotional, but one speaks with an icy distance. “I wasn’t particularly upset about Chris. Shit happens,” he says.

For all these virtues, there is still the sense of a film stretching hard to make it to feature length. Chris’s wife is here to provide some wider background. The music pounds unnecessarily. The divers talk delicately around one particular topic that remains conspicuously unmentioned until it is mentioned.

We’ll say no more.

Opens on April 5th