Amid all the mathematical bluster that threatens to seriously bog down Logopolis, the little character moments drag it out of the muck. Just such a moment occurs in this episode when The Doctor is trapped inside a shrunken TARDIS, crumpled on the floor, a defeated man, relying entirely on help from the outside; help that may never come.
In his dark moments, he starts quoting Huxley, and then sums up his current situation: "I'm an ignorant old Doctor, and I've made a mistake". It's a rare admission from this Fourth Doctor, who has always been a tower of power, always in control of his situation, but a Doctor's whose rare moments of insecurity make him such an endearing character. This scene in Logopolis reminds of his lamenting his advancing age in Pyramids of Mars, or his admission of his tragic mistake in Horror of Fang Rock.
This is a Doctor approaching the end of his days, as evidenced by the increased appearances of the Watcher. We'll find out for certain who the mysterious figure in white is in Episode Four, but for now he appears almost as a portent of doom, the grim reaper without a scythe.
Plot points about Charged Vacuum Emboitments and Block Transfer Computation are becoming less and less important (and yet more confusing at the same time). For a generation of Doctor Who fans, of which I was one, the unthinkable was about to happen.