It's been a couple of decades On their route to Sam's next piano recital, Sam (Colin Firth) and Tusker (Stanley Tucci), who is suffering from early onset dementia, are mixing work and pleasure by traveling around the Lake District, visiting relatives and old haunts. With each passing second becoming increasingly valuable, Sam finds Tusker is keeping a secret from him.

Supernova is, at its core, a British road movie, a cinematic rarity. Sam (Colin Firth) and Tusker (Stanley Tucci), a lapsed concert pianist, are driving through the Lake District in a campervan en route to Sam's comeback recital. Listening to Donovan and David Bowie, bantering about the opaque quality of the Shipping Forecast to an outsider, and debating map-reading vs. sat nav (Tusker wants the latter turned off because it sounds like Margaret Thatcher). When Tusker goes missing at a Spar service station, Sam bombs along country lanes in search of him, it becomes evident that something is wrong. Tusker has had early onset dementia for two years, a condition that is revealed and explored via the (very writerly) device of recording a regular cassette — "Welcome to Dementia Hour On Radio 4" — in which the couple discuss their sentiments (“Can you see it's getting worse?” Tusker asks bluntly). Tusker requiring assistance to put on a sweater or being unable to conjure up the phrase "triangle"; Sam silently composing himself in the confines of a campervan loo are all signs of the sickness.

Firth and Tucci build a lived-in connection with ease and warmth, whether it's reminiscing about the first time Tusker uttered "I love you" (in Tusker's tiny car) or the unintentional scratching of a forearm ("I enjoy it when you do that"). Tucci is in quixotic, mesmerizing form — watch out Tusker's wonder when he talks about astronomy — employing wit and cheek to mask a wellspring of emotions, a man who badly wants to be remembered for who he was, not what he is about to become.