This "guys on a mission to blow something up in wartime" film has the benefit of being based on an actual real life mission that was actually successful (many of them were disasters) - the Norwegian heavy water sabotage in World War Two, in particular Operation Gunnerside, the destruction of a chamber and the sinking of the SF Hydro.
This means the film essentially has two climaxes - blowing up a chamber, then sinking a ferry - with a lot of skiing and escaping in between. It also means the film has a repetitive feel, despite some great moments. There's not really a sense of progression or dramatic build, which is problematic when you've got a two hour plus running time and is the chief flaw of this otherwise decent enough movie. (Mind you, I pretty much never met a guys on a mission film I didn't like.)
Richard Harris is the local Norwegian resistance fighter who recruits scientist Kirk Douglas to help him on the mission. Both actors are effective - Harris had a glowering enigmatic intensity, a brooding quality, that made him very attractive to casting directors in action movies. You don't think of him as an action star but he made an awful lot of them. Kirk Douglas does his Kirk Douglas thing, jutting out the chin and never being too convincing, but he's Kirk Douglas so you go with it.
It's a shame neither have much of a character to play. Both are tough and quite ruthless - Douglas is a scientist but a rugged one who adapts to war time easily. The characters argue over some issue but the conflict feels forced. Douglas is given an ex wife, Ulla Jacobsson - they were meant to have this great sex life but they are together in this very unsexy scene wearing daggy pyjamas. I wish they'd really gone with Douglas being a nerdy man of science and Harris the man of action, and used Jacobsson's sexiness more. The Nazis are stock Nazis - either shouting generals or doltish henchmen - but Michael Redgrave is effective in support and has a memorable death scene.
There are some stunning visuals - I have a soft spot for action films set in the snow. Director Anthony Mann was still in fine fore - there's great bits like the commandos watching a plane of British come to their aid and then crash, and the commandos skiing silently down the snow, and when they come across the Norwegian skier who may or may not be trustworthy. Blowing up the ferry at the end felt more conventional.
I get the feeling with this movie they never quite got the story right and as a result concentrated on "bits" and scenes. It doesn't flow satisfactorily but has its moments.