Tenth Doctor
The Doctors discover that the same TARDIS at different points in time have collided because the Tenth Doctor left his shields down. This creates a paradox that will cause a black hole strong enough to swallow the entire universe. The Tenth Doctor counters it with a supernova, a solution he remembers seeing himself perform in this same incident (a predestination paradox), making the Fifth Doctor realise the Tenth Doctor really is his future self. The Tenth Doctor reminisces as the Fifth Doctor begins to fade into a separate timeline, revealing various traits from his fifth incarnation that he retained for his tenth, and wishes him well.