For the first two-thirds this wartime drama (thriller would be stretching it) ambles along amiably enough. Intrigue revolves around an English scientist on a Scottish estate; the government is concerned that his isolated work runs risks of secrets passing to the enemy and would much rather he moved to London. Sure enough, various suspicious parties converge on the titular cottage and accompanying manor house. This plays as (not especially uproarious) light comedy romance during the opening acts, before transitioning quite effortlessly into high drama.
George Cole makes his first screen appearance (16, but looking younger) as a resourceful, smart-alec evacuee. Unsurprisingly, his mentor Alastair Sim also stars; slightly trimmer than in his more familiar roles but just as reassuringly chrome-domed. John Mills plays a dashing RAF lieutenant; bloody John Mills.
What makes this worth a look is some genuinely surprising misdirection and a fairly full-on denouement, which includes a contender for all-time worst screen death.