In Dominic Cooke’s The Courier, Cumberbatch plays an unremarkable British businessman who gets roped into an elaborate Cold War espionage scheme in the early 1960s.

Cumberbatch is quite appealing as Greville Wynne, an unassuming, deferential engineer and salesman whose frequent business trips to Eastern Europe in the 1950s prompted American and British spy services to enlist him in ferrying messages from Colonel Oleg Penkovsky (Merab Ninidze), a higher-up in the USSR’s military intelligence agency. These efforts led to key intelligence about the Soviet military buildup in Cuba, which brought the world to the brink of nuclear war in 1962 but also wound up establishing a direct line between Moscow and Washington, thus presumably saving the planet from future calamities. It’s one of the most remarkable espionage stories of the 20th century, but The Courier wisely presents it as a tale of personal loyalty instead of geopolitics or spycraft.

Cooke and screenwriter Tom O’Connor even give Greville and Oleg’s relationship the subtle aura of a clandestine romance. After he delivers Oleg to the hotel room where his CIA handler, Emily (Rachel Brosnahan), is waiting, the door slowly, agonisingly shuts on Greville’s face, and it’s not hard to imagine him as a dismissed lover. What’s more, as his trips to Moscow become more frequent, Greville’s impatience with his family back home grows, and his wife, Sheila (Jessie Buckley) starts to suspect there might be another woman in the picture.

This notion of the espionage connection as a kind of love affair is not a cheap or meaningless one. The platonic love that develops between these two men does become critical later in the story, as their situation becomes more desperate. The film builds suspense not on Oleg’s revelations — even the Cuban Missile Crisis is mostly treated as background noise — but on the increasingly codependent nature of the relationship between him and Greville. One becomes the key to the other, which makes every turn in their friendship that much more tense.

Cumberbatch is terrific, but the real attraction here is the great Georgian actor Ninidze, who can relay an entire novel’s worth of information with just a couple of glances. The Courier is a serviceable espionage drama and history lesson, but whenever these two actors are onscreen together, it approaches the sublime.