Formula 1 finds itself at a crossroads, a junction in history, as it embarks on a marathon 2021 World Championship.
But whether it runs to full length or not, it is set to be a momentous year.
Lewis Hamilton, the dominant figure of his generation, could end it with an all-time record eighth drivers' title. But the year may also be the Mercedes driver's swansong, as he starts it with a contract only for 2021, and has so far equivocated on whether he wants to continue after that.
In addition, the sport embarks on a new era, with the introduction of a budget cap among a series of steps aimed at making the field more competitive. And it is the last year before the introduction of a major regulation change that will herald a whole new kind of grand prix car.
To save money, teams have been forced to carry over large parts of their cars from last season. But a rule change aimed at slowing them down - or at least preventing them from getting significantly faster - means there remains every chance of a change in the competitive order.
Mercedes start the year as favourites after seven consecutive world title doubles, but Red Bull and engine partner Honda have pulled out all the stops in the final year of the Japanese company's time in F1.
Britain's Hamilton starts the year already installed as the most successful racing driver in history. His seventh title last season matched the achievement of Michael Schumacher, while the 36-year-old stands alone at the top of the lists of wins and pole positions.
Starting the year with 95 victories, Hamilton could well pass the century mark in 2021, an unprecedented, mind-bending achievement that not so long ago was thought impossible.
On paper, the eighth title appears close to a gimme. Hamilton and Mercedes dominated last year, his team-mate Valtteri Bottas has not yet proved capable of challenging him consistently over a season and changes to cars have been restricted over the winter.
In reality, though, it may not be the walkover many would have predicted a couple of months ago.
To prevent further escalation in cornering speeds, a couple of significant rule changes have been made at the back of the car, cutting away parts of the floor and diffuser, both critical in producing aerodynamic downforce.
Mercedes technical director James Allison describes these as "a sort of regulatory vandalism that actually knocked the performance of the car right back about a year and a half". They change the airflow requirements at the rear, forcing teams to reassess how to claw back the lost downforce.
Pre-season testing did not go well for the world champions, Mercedes struggling with a nervous and skittish car, and Hamilton concluding they had "a lot of work to do".
But one has to assume they will eventually get on top of this and emerge as the usual formidable force, as they did in 2017 and 2018, when they started the season on the back foot against a generally quicker Ferrari.
Red Bull, by contrast, had what motorsport director Helmut Marko described as their best ever pre-season test. Could this be the year that Red Bull, with an all-new Honda engine brought forward from 2022 because of the Japanese company's withdrawal from F1 at the end of 2021, finally go toe-to-toe with Mercedes?
If they can, the prospect of a straight fight between Hamilton and 23-year-old Max Verstappen, one of the two leading figures of the new generation of drivers, is mouth-watering.
The 2021 season starts with a significant reshuffle of the driver line-ups at the leading teams. Of those who finished in the top six of the championship last season, only Mercedes have the same two drivers this year as in 2020.
And each combination is full of intrigue, meaning and consequence.
Sebastian Vettel - fresh from a bruising two years alongside Charles Leclerc at Ferrari - has found a new home at the renamed Aston Martin team, as team-mate to Lance Stroll, son of the owner.
It has not been a great last seven years for Vettel. He has been outpaced at Red Bull by Daniel Ricciardo, been less superior to Kimi Raikkonen as a team-mate at Ferrari than Fernando Alonso was, made a series of high-profile and damaging driving errors, compromised at least one and possibly two chances to win the title, been outclassed by Leclerc, and been deemed surplus to requirements at Ferrari.
As a consequence, he has some reputation rebuilding to do, regardless of his four world titles and 53 race victories.
Alonso, meanwhile, is back in F1 after two years racing in other categories, driving for the Alpine team, formerly Renault.
Source: BBC Sport.